fix: audit fixes for PR #138 JSONL-triggered SQLite sync

[ProfSynapse]

Summary

Post-merge audit of PR #138 identified 1 HIGH, 4 MEDIUM, and 6 LOW findings across architecture, backend, frontend, and test coverage. This PR addresses the HIGH and all MEDIUMs:

• H1: SyncCoordinator.sync() concurrency guard — prevents overlapping sync runs from watcher + periodic timer applying duplicate events
• M1: Self-write suppression changed from consume-on-match to TTL-based expiry — handles shard rotation (two vault events per write)
• M2: Expired suppression entry sweep in scheduleDispatch — prevents unbounded Map growth in long sessions
• M3: WorkspacesTab skips full re-render during detail form editing when sync arrives for unrelated workspaces — prevents unsaved form data loss
• M4: ConversationList preserves scrollTop across sync-triggered re-renders
• M5+L4: 8 new tests — explicit create/delete vault event coverage (primary Obsidian Sync trigger) and error resilience (dispatch survives onChange throws)

Test plan

• All 27 JsonlVaultWatcher tests pass (19 existing + 8 new)
• Build passes clean (lint + tsc + esbuild + connector)
• Manual: verify workspace detail form edits survive external sync events
• Manual: verify conversation list scroll position preserved during sync
• Manual: verify no duplicate sync runs under load (check logs for overlapping sync entries)

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