feat(offline): IndexedDB task-config store with sync-on-reconnect (#421)

[Rufai-Ahmed]

Summary

Adds an offline-first, IndexedDB-backed store for task configurations that automatically syncs when connectivity is restored (#421).

What's included

• src/lib/offline/taskConfigDb.ts — a small, dependency-free promisified wrapper over IndexedDB: saveTaskConfig (stored as pending), getTaskConfig, getAllTaskConfigs, getPendingConfigs, markConfigSynced, deleteTaskConfig, and syncTaskConfigs(push).
• src/lib/offline/useOfflineTaskConfig.ts — a React hook that persists configs offline and flushes them through an injected push function whenever the app comes (back) online. Exposes configs, pendingCount, online, syncing, saveConfig, sync.

Design notes

• Reuses the existing useOnlineStatus primitive (src/lib/network) rather than reinventing online detection.
• Sync is per-item resilient: a failed push leaves that config pending for the next attempt and never aborts the rest of the batch.
• The transport (push) is injected, so the store is decoupled from the actual task-registration/RPC call.

Testing

• 9 Jest tests across the store and hook (offline save → reconnect → sync, partial-failure handling, immediate online sync), all passing.
• Added a guarded structuredClone polyfill to jest.setup.js (jsdom doesn't expose it; fake-indexeddb needs it) and fake-indexeddb as a dev dependency.

Follow-ups

Wire push to the real task-registration call and surface pendingCount in the existing OfflineStatusBar.

Closes #421

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