feat: safe reconnect for collaborative editor

[pimaster900]

• Add useCollaborativeEditor hook with singleton provider registry (one Y.Doc + WebsocketProvider per roomId/userId pair, refcounted)
• Add PendingUpdateQueue utility that buffers local Yjs updates while disconnected, deduplicates by content key, and caps at 100 entries
• Replay queued updates into the shared doc on reconnect (FIFO order)
• Expose hasConflict flag when remote state advanced while offline
• Expose reconnectStatus ('connected' | 'disconnected' | 'reconnecting')
• Full teardown on last consumer unmount (disconnect + destroy)
• Add ReconnectBanner component with accessible live regions (role=status / role=alert), actionable copy, and responsive desktop/mobile layout
• Update CollaborativeCanvas to use new hook and banner; all existing node/edge/export/awareness behaviour preserved
• Add vitest unit tests for hook (singleton, queue, conflict, teardown, reconnect(), dismissConflict())
• Add vitest integration tests simulating disconnect → offline edits → reconnect → replay without duplicates

closes #935

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