fix(tasks): revalidate stale MMKV task cache on mount

[devprom6]

Summary

The offline task feed cache (MMKV) had no TTL and no revalidation trigger. If the store held tasks from a previous session, useTaskFeed served them indefinitely and never called the API again — even on a fresh app start with connectivity. New tasks, status changes, and reward updates were invisible until a manual pull-to-refresh.

This adds a tasksLastFetchedAt timestamp and a mount-time stale check that silently revalidates the cache when it's older than TASK_FEED_STALE_MS (default 5 min).

Changes

src/store/taskStore.ts

• Added tasksLastFetchedAt: string | null to TaskState and PersistedTaskSlice (persisted to MMKV).
• setTasks stamps tasksLastFetchedAt with the current ISO time on every page-1 fetch.
• sanitizePersistedTaskState passes tasksLastFetchedAt through (reset to null when the cache is empty).
• reset() clears tasksLastFetchedAt.

src/hooks/useTaskFeed.ts

• loadTasks(pageNum, silent = false) — silent mode skips the loading skeleton (setLoading).
• Mount effect now computes isStale from tasksLastFetchedAt and, when the warm cache is older than TASK_FEED_STALE_MS, triggers a silent background fetch (loadTasks(1, true)).
• selectedTask is never clobbered during revalidation because setTasks only replaces tasks.

src/config/taskFeedCache.ts (new)

• Exports TASK_FEED_STALE_MS = 5 * 60 * 1000 as a separately-mockable module so tests can override the threshold.

Tests

• useTaskFeed.test.ts / useTaskFeedRefetch.test.tsx: within-TTL → no fetch; past-TTL → silent fetch with no loading skeleton; selectedTask preserved after revalidation.
• stores.test.ts: updated assertions for the new persisted field.

Acceptance Criteria

• taskStore gains a tasksLastFetchedAt field, persisted to MMKV.
• useTaskFeed triggers a background fetch on mount when tasksLastFetchedAt is older than TASK_FEED_STALE_MS (5 min default), even if the store has tasks.
• The background fetch does not show a loading skeleton.
• selectedTask is not clobbered during background revalidation.
• Tests cover: (a) warm cache within TTL → no fetch, (b) warm cache past TTL → background fetch, (c) selectedTask preserved after revalidation.

Test plan

• npm test — all task feed / store suites pass (48 tests).
• npx tsc --noEmit and npx eslint clean.

Out of scope

• Server-sent events / real-time task updates.
• Per-filter TTL (all filters share the same tasksLastFetchedAt).

Closes #150

[cybermax4200]

Excellent work!