fix(storage): flush pending debounced writes on unmount instead of discarding them

[Matthew-Selvam]

Summary

The useStorageSync unmount cleanup cleared the debounce timer without flushing, and pendingCollectionsRef — a per-mount useRef — was dropped with the component. An edit made within the 500ms debounce window of navigating away existed only in localStorage; the next mount's pullFromServer() then unconditionally overwrote localStorage with server data, destroying the write permanently and silently.

Fix

The cleanup now flushes whatever is pending before tearing down:

• Guarded on serverModeRef — in local mode the queue is always empty and the PUT would be pointless work.
• Guarded on a non-empty pending set — no spurious push when nothing is queued.
• A failed post-teardown flush still re-queues its collections (existing behavior), and the existing !mountedRef.current guard already keeps it from arming stray retry timers — so the previous fix for the dead-hook retry loop is unaffected.

Test plan

• New: a pending debounced write is flushed on unmount, not discarded — edit dispatched inside the debounce window, unmount, assert the PUT lands
• New: unmount with nothing pending does not push — asserts the flush is conditional
• Existing a push that fails after unmount does not leave a timer running still passes (in-flight push ≠ pending push; no double-send)

Found during a broader code review of the storage sync layer; no issue existed yet for it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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