feat: implement PostHog event tracking plan
Add trackEvent() calls for all user actions in the tracking plan:
- Vault: vault_opened (with has_git, note_count), vault_switched
- Notes: note_created (with has_type, creation_path), note_deleted,
note_trashed, note_archived, note_favorited, note_unfavorited
- Git: commit_made, sync_triggered
- Search: search_used
- Editor: raw_mode_toggled, wikilink_inserted
- Types & Views: type_created, view_created
- Telemetry: telemetry_opted_in, telemetry_opted_out
All events gated by PostHog initialization (only fires when opted in).
No PII in any event properties — counts, booleans, and enums only.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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