feat: add navigation-safe frontend reorder coordinator and save-state UX

[devin-ai-integration]

Closes #979. Part of epic #975.

Summary

Replaces Browse's component-local reorder persistence with a navigation-safe coordinator (frontend/src/tileOrdering.ts) owned above SortableTileGrid, plus a save-state indicator.

• Coordinator (TileOrderingCoordinator, module-level singleton): per-scope state machine idle → dirty → saving → (dirty-while-saving) → saved | conflict | error. Every accepted drag applies locally and is reported; drops during an active save are queued and coalesced to the newest snapshot — never discarded. Persists via the atomic PUT /api/tile-order CAS contract from Implement an atomic, revisioned tile-order API with set-based updates #978, seeding the revision with a deduped GET on first save (scope enters saving during seeding; seed failures emit failed telemetry).
• Conflict handling: 409 surfaces "Order changed elsewhere" with the authoritative order; a 400 membership rejection is treated the same way (refreshed via GET) instead of retrying stale membership forever. Failures keep the newest local intent retryable; success applies the authoritative response directly (no category-tree refresh) and never shows "saved" while newer local changes exist.
• Navigation safety: state survives grid unmount/remount; per-scope generation fencing (claimGeneration) prevents an unmounted grid's callbacks from overwriting its replacement; a beforeunload guard warns while unsaved order exists.
• UI: ReorderStatusIndicator (Unsaved order / Saving order… / Order saved / Order changed elsewhere → Refresh / Could not save order → Retry) rendered above the Browse grid; useTileOrdering React binding via useSyncExternalStore.
• Grid: optional tileOrdering prop — coordinator mode applies drags locally and reports refs; legacy persistence untouched when absent (Manage Categories stays legacy until Migrate Manage Categories to the shared ordering workflow and complete end-to-end verification #982). No collision-detection changes.
• Telemetry: emits queued/coalesced/submitted/committed/conflicted/failed per the Add reorder operation correlation, timing, and diagnostic telemetry #977 contract.
• Docs: frontend coordinator section added to docs/tile-ordering.md.

Testing

• npm test: 88 files, 1358 passed (3 expected fail).
• New tests: tests/tileOrdering.test.ts (14 coordinator tests incl. queue-during-save, coalescing, unmount/remount survival, generation fencing, 409 and 400 conflict paths, seeding failure telemetry, 20 rapid reorders), tests/components/SortableTileGridCoordinator.test.tsx (4), tests/components/ReorderStatusIndicator.test.tsx (7).
• npm run lint, npm run build, prettier clean.

Needs human verification

• Indicator UX pass: persistent "Order saved" readout, copy after conflict-Refresh, and self-inflicted 409 wording during the mixed legacy/coordinator window (closes in Migrate Manage Categories to the shared ordering workflow and complete end-to-end verification #982).
• Drag feel-testing per docs/drag-and-drop.md policy (no collision logic changed).

Link to Devin session: https://app.devin.ai/sessions/414754bd88374dc693f47100ac269e78

[devin-ai-integration]

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[devin-ai-integration]

Devin Review found 7 potential issues.

[devin-ai-integration]

Devin Review found 3 new potential issues.

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Devin Review found 4 new potential issues.