feat(plugins): add drag-and-drop reordering to plugin catalog (#604)

[annukumar123]

Summary

Implements HTML5 drag-and-drop reordering for the plugin catalog workspace, allowing users to customize and persist their preferred plugin layout across sessions (#604).

Changes Made

• Drag-and-Drop UX (PluginsPanel.jsx):
  • Added HTML5 draggable handlers (onDragStart, onDragOver, onDrop) to plugin chips.
  • Implemented visual feedback (cursor-grab, cursor-grabbing, and dashed outline during drag state).
  • Added safe guards for e.dataTransfer properties to handle mock/test environments cleanly.
• State Persistence (PluginsPanel.jsx):
  • Reordered plugin ID lists are saved to localStorage under plugins-panel-order:${sessionId}.
  • On panel load, incoming API plugins are sorted according to the stored sequence.
• Unit Testing (PluginsPanel.test.jsx):
  • Added test suite covering initial rendering, drag-and-drop interaction, order persistence, and restoring custom order on mount.

Verification

• Ran Vitest suite with JSDOM: npx vitest run src/components/PluginsPanel.test.jsx
• Result: 4 passed (100%) with zero unhandled errors or warnings.

Fixes #604

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