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>

src/hooks/useAppKeyboard.ts

@@ -1,5 +1,6 @@
 import { useEffect } from 'react'
 import type { ViewMode } from './useViewMode'
+import { trackEvent } from '../lib/telemetry'
 
 interface KeyboardActions {
   onQuickOpen: () => void
@@ -104,6 +105,7 @@ export function useAppKeyboard({
       // Cmd+Shift+F: full-text search (distinct from Cmd+F browser find)
       if ((e.metaKey || e.ctrlKey) && e.shiftKey && e.key === 'f') {
         e.preventDefault()
+        trackEvent('search_used')
         onSearch()
         return
       }