Introduce desktop shell layout for web dashboard

lib/shell/app_shell.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+
+import 'desktop_shell.dart';
+import 'shell_destination.dart';
+
+class AppShell extends StatelessWidget {
+  /// Matches Figma desktop artboards (~1700px). Sidebar shell activates at
+  /// this width; mobile bottom navigation remains below it.
+  static const double desktopBreakpoint = 1024;
+
+  final Widget mobile;
+  final Widget child;
+  final ShellDestination selectedDestination;
+  final ValueChanged<ShellDestination>? onDestinationSelected;
+
+  const AppShell({
+    super.key,
+    required this.mobile,
+    required this.child,
+    this.selectedDestination = ShellDestination.dashboard,
+    this.onDestinationSelected,
+  });
+
+  static bool isDesktop(BuildContext context) {
+    return MediaQuery.sizeOf(context).width >= desktopBreakpoint;
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (isDesktop(context)) {
+      return DesktopShell(
+        selectedDestination: selectedDestination,
+        onDestinationSelected: onDestinationSelected,
+        child: child,
+      );
+    }
+
+    return mobile;
+  }
+}

lib/shell/desktop_shell.dart

@@ -0,0 +1,41 @@
+import 'package:flutter/material.dart';
+
+import 'shell_destination.dart';
+import 'widgets/shell_navigation_rail.dart';
+
+class DesktopShell extends StatelessWidget {
+  final Widget child;
+  final ShellDestination selectedDestination;
+  final ValueChanged<ShellDestination>? onDestinationSelected;
+
+  const DesktopShell({
+    super.key,
+    required this.child,
+    this.selectedDestination = ShellDestination.dashboard,
+    this.onDestinationSelected,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final colorScheme = Theme.of(context).colorScheme;
+
+    return Scaffold(
+      backgroundColor: Theme.of(context).scaffoldBackgroundColor,
+      body: Row(
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          ShellNavigationRail(
+            selectedDestination: selectedDestination,
+            onDestinationSelected: onDestinationSelected,
+          ),
+          VerticalDivider(
+            width: 1,
+            thickness: 1,
+            color: colorScheme.outlineVariant,
+          ),
+          Expanded(child: child),
+        ],
+      ),
+    );
+  }
+}

lib/shell/shell_destination.dart

@@ -0,0 +1,54 @@
+import 'package:flutter/material.dart';
+
+enum ShellDestination {
+  dashboard,
+  calendar,
+  workspace,
+  chat,
+  profile;
+
+  String get label {
+    switch (this) {
+      case ShellDestination.dashboard:
+        return 'Dashboard';
+      case ShellDestination.calendar:
+        return 'Calendar';
+      case ShellDestination.workspace:
+        return 'Workspace';
+      case ShellDestination.chat:
+        return 'Chat';
+      case ShellDestination.profile:
+        return 'Profile';
+    }
+  }
+
+  IconData get icon {
+    switch (this) {
+      case ShellDestination.dashboard:
+        return Icons.dashboard_outlined;
+      case ShellDestination.calendar:
+        return Icons.calendar_today_outlined;
+      case ShellDestination.workspace:
+        return Icons.work_outline;
+      case ShellDestination.chat:
+        return Icons.chat_bubble_outline;
+      case ShellDestination.profile:
+        return Icons.person_outline;
+    }
+  }
+
+  IconData get selectedIcon {
+    switch (this) {
+      case ShellDestination.dashboard:
+        return Icons.dashboard;
+      case ShellDestination.calendar:
+        return Icons.calendar_today;
+      case ShellDestination.workspace:
+        return Icons.work;
+      case ShellDestination.chat:
+        return Icons.chat_bubble;
+      case ShellDestination.profile:
+        return Icons.person;
+    }
+  }
+}

lib/shell/widgets/shell_navigation_rail.dart

@@ -0,0 +1,192 @@
+import 'package:flutter/material.dart';
+
+import '../shell_destination.dart';
+
+/// Fixed-width sidebar aligned with Figma shell (~288px).
+class ShellNavigationRail extends StatelessWidget {
+  static const double width = 288;
+
+  final ShellDestination selectedDestination;
+  final ValueChanged<ShellDestination>? onDestinationSelected;
+
+  const ShellNavigationRail({
+    super.key,
+    required this.selectedDestination,
+    this.onDestinationSelected,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final colorScheme = theme.colorScheme;
+
+    return SizedBox(
+      width: width,
+      child: Material(
+        color: colorScheme.surface,
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: [
+            Padding(
+              padding: const EdgeInsets.fromLTRB(24, 22, 24, 16),
+              child: Row(
+                children: [
+                  Container(
+                    width: 48,
+                    height: 48,
+                    decoration: BoxDecoration(
+                      color: colorScheme.primary.withValues(alpha: 0.15),
+                      borderRadius: BorderRadius.circular(12),
+                    ),
+                    alignment: Alignment.center,
+                    child: Text(
+                      'E',
+                      style: theme.textTheme.titleLarge?.copyWith(
+                        color: colorScheme.primary,
+                        fontWeight: FontWeight.bold,
+                      ),
+                    ),
+                  ),
+                  const SizedBox(width: 12),
+                  Text(
+                    'Ell-ena',
+                    style: theme.textTheme.titleMedium?.copyWith(
+                      fontWeight: FontWeight.w600,
+                    ),
+                  ),
+                ],
+              ),
+            ),
+            Divider(
+              height: 1,
+              thickness: 1,
+              color: colorScheme.outlineVariant,
+              indent: 24,
+              endIndent: 24,
+            ),
+            const SizedBox(height: 12),
+            Expanded(
+              child: ListView(
+                padding: const EdgeInsets.symmetric(horizontal: 12),
+                children: [
+                  for (final destination in ShellDestination.values)
+                    _ShellNavItem(
+                      destination: destination,
+                      isSelected: destination == selectedDestination,
+                      onTap: onDestinationSelected == null
+                          ? null
+                          : () => onDestinationSelected!(destination),
+                    ),
+                ],
+              ),
+            ),
+            Divider(
+              height: 1,
+              thickness: 1,
+              color: colorScheme.outlineVariant,
+              indent: 24,
+              endIndent: 24,
+            ),
+            Padding(
+              padding: const EdgeInsets.fromLTRB(24, 16, 24, 24),
+              child: Row(
+                children: [
+                  CircleAvatar(
+                    radius: 21,
+                    backgroundColor:
+                        colorScheme.primary.withValues(alpha: 0.15),
+                    child: Icon(
+                      Icons.person,
+                      size: 20,
+                      color: colorScheme.primary,
+                    ),
+                  ),
+                  const SizedBox(width: 12),
+                  Expanded(
+                    child: Column(
+                      crossAxisAlignment: CrossAxisAlignment.start,
+                      children: [
+                        Text(
+                          'Account',
+                          style: theme.textTheme.bodyMedium?.copyWith(
+                            fontWeight: FontWeight.w600,
+                          ),
+                          overflow: TextOverflow.ellipsis,
+                        ),
+                        Text(
+                          'Signed in',
+                          style: theme.textTheme.bodySmall?.copyWith(
+                            color: colorScheme.onSurfaceVariant,
+                          ),
+                          overflow: TextOverflow.ellipsis,
+                        ),
+                      ],
+                    ),
+                  ),
+                ],
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class _ShellNavItem extends StatelessWidget {
+  final ShellDestination destination;
+  final bool isSelected;
+  final VoidCallback? onTap;
+
+  const _ShellNavItem({
+    required this.destination,
+    required this.isSelected,
+    this.onTap,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final colorScheme = theme.colorScheme;
+    final foregroundColor = isSelected
+        ? colorScheme.primary
+        : colorScheme.onSurfaceVariant;
+
+    return Padding(
+      padding: const EdgeInsets.only(bottom: 4),
+      child: Material(
+        color: isSelected
+            ? colorScheme.primary.withValues(alpha: 0.12)
+            : Colors.transparent,
+        borderRadius: BorderRadius.circular(12),
+        child: InkWell(
+          onTap: onTap,
+          borderRadius: BorderRadius.circular(12),
+          child: Padding(
+            padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 14),
+            child: Row(
+              children: [
+                Icon(
+                  isSelected ? destination.selectedIcon : destination.icon,
+                  size: 24,
+                  color: foregroundColor,
+                ),
+                const SizedBox(width: 12),
+                Text(
+                  destination.label,
+                  style: theme.textTheme.bodyLarge?.copyWith(
+                    color: isSelected
+                        ? colorScheme.onSurface
+                        : colorScheme.onSurfaceVariant,
+                    fontWeight:
+                        isSelected ? FontWeight.w600 : FontWeight.w500,
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}