* perf(app): reduce composer draft writes
Persist draft checkpoints every 200 ms during sustained typing while keeping the latest in-memory draft current. A trailing write stores the newest checkpoint when typing stops.
* test(app): cover draft persistence checkpoints
Keep the throttle policy deterministic at its storage boundary, including latest-value coalescing and pending-write cancellation when drafts are cleared.
* fix(app): flush pending drafts on background
Flush the latest throttled checkpoint when the app leaves the foreground and report asynchronous persistence failures. Extend the storage-boundary tests across consecutive intervals and explicit lifecycle flushing.
