feat(ui): compact morphing sync status bar with event log

Redesign the status bar into a progressive-disclosure control: idle dot →
"Sync now" button → spinner → timestamped event-log popup.

- new syncEventLog module (+ pure -core) with localStorage persistence
  (mailbrus_sync_events, 2000-line FIFO, 100ms dedup, run archival)
- syncState captures lifecycle/index/sync_finished frames and emits
  sync_completed/sync_failed
- StatusBar: three-state morph, 15-event scrollable log, history, copy-log,
  clear-history, error-state dot
- unit tests for the event-log core logic

Author: antono