feat(#660): Implement offline-first conflict resolution for concurrent edits

[zeemscript]

Summary

Implements offline-first conflict resolution for concurrent edits, addressing the scenario where users edit notes, quiz drafts,
or profile information while offline and then reconnect to find their mutations conflict with server-side changes.

Changes

• Request Queue Enhancement: Added lastKnownVersion, clientTimestamp, entityType, and entityId fields to QueuedRequest to track version metadata for conflict detection
• 409 Conflict Handling: Updated axios interceptor to detect 409 responses, extract conflict data from both request headers and server response, and dispatch to the conflict store
• Conflict Store: Created Zustand store (conflictStore.ts) to manage conflict queue, active conflict state, resolution history, and modal visibility
• Conflict Resolution UI: Added ConflictResolutionModal component with:
  • Diff view showing local vs server changes with highlighting
  • Tab navigation between "Differences", "Your Version", and "Server Version" views
  • "Keep Mine" (override server) and "Use Server" (discard local) resolution buttons
  • Accessibility labels and dark mode support
• App Integration: Integrated ConflictResolutionModal into the root layout for global conflict resolution

Test Plan

• Run conflict detection tests (npm test -- --testPathPattern="conflict")
• Test offline mutation that conflicts with server change triggers conflict UI
• Verify user can choose local or server version
• Verify conflict UI shows readable diff between versions
• Verify resolved conflict syncs chosen version back to server

Closes #660

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