fix: resolve DataTable pagination dropdown sync and AppShell border styling issues

[christoph2806]

Fix: DataTable Pagination Dropdown Sync and AppShell Border Styling Issues

🐛 Issues Fixed

1. DataTable Pagination Dropdown Synchronization

• Problem: Pagination dropdown could get out of sync with actual page size when custom page sizes weren't included in pageSizeOptions
• Impact: Users would see incorrect selected values in the dropdown, causing confusion about current pagination state

2. AppShell Border Styling Inconsistencies

• Problem: Hard-coded color values (#000000, border-gray-200) instead of design system tokens
• Impact: Inconsistent theming across components, poor dark mode support

🔧 Solutions Implemented

DataTable Pagination Fixes

• ✅ Enhanced smart pagination configuration to ensure pageSize is always included in pageSizeOptions
• ✅ Implemented automatic deduplication using Set to prevent duplicate values in dropdown
• ✅ Applied consistent sorting to maintain proper option order
• ✅ Fixed both controlled and auto-detected pagination scenarios

AppShell Styling Fixes

• ✅ Replaced hard-coded colors with design system tokens (hsl(var(--border)), hsl(var(--foreground)))
• ✅ Updated all AppShell components: AppShell, AppShellTopBar, ContentWrapper
• ✅ Fixed MarkdownEditor text color consistency
• ✅ Improved theme compatibility (dark/light mode support)

🧪 Testing

• ✅ All existing tests pass (1028 tests passed, 7 skipped)
• ✅ Linting passes with no errors
• ✅ Build successful
• ✅ Manual verification of pagination dropdown sync
• ✅ Visual verification of consistent borders across themes

📝 Files Changed

• packages/ui-kit/src/components/data-display/DataTable/DataTable.tsx - Pagination sync fixes
• packages/ui-kit/src/layout/AppShell/AppShell.tsx - Footer border fix
• packages/ui-kit/src/layout/AppShell/AppShellTopBar.tsx - Top bar border fix
• packages/ui-kit/src/layout/AppShell/ContentWrapper.tsx - Content wrapper border fixes
• packages/ui-kit/src/components/primitives/MarkdownEditor/MarkdownEditor.tsx - Text color fix

🎯 Verification Steps

1. Create a DataTable with custom pageSize not in default options → Dropdown shows correct value
2. Switch between page sizes → No sync issues
3. Test AppShell components in light/dark themes → Consistent borders
4. Verify MarkdownEditor disabled state → Proper text color

📋 Closes

• Closes [Bug] AppShell uses hardcoded black borders instead of theme-aware colors #60 - [Bug] AppShell uses hardcoded black borders instead of theme-aware colors
• Closes Bug: DataTable Pagination Dropdown Synchronization Issue #61 - Bug: DataTable Pagination Dropdown Synchronization Issue

Fixes pagination dropdown synchronization issues and AppShell styling inconsistencies identified during component testing.