feat: frame-based positioning and mark system foundation

Core Changes:
- Fix SearchView to use start_frame/end_frame directly (no time*fps conversion)
- Add hard_delete support to delete_trace API
- VideoPlayer: Main timeline + Mark system foundation
- Proxy: Add local routes for auth, media, identity-matches, cluster-results
- Add .gitignore to exclude build artifacts and dependencies

Design Documents:
- Multi-track Mark system design (.opencode/plans/)
- Video editing positioning standards research

Files Modified:
- src/views/SearchView.vue: Frame positioning, ensureMinDuration (240 frames)
- src/views/PeopleView.vue: batchDeleteGroups with hard_delete
- src/api/index.ts: delete_trace with hard_delete body
- src/components/VideoPlayer.vue: Timeline + Mark UI
- src-tauri/src/proxy.rs: New local routes
- AGENTS.md: Update documentation

src/api/config.ts

@@ -2,5 +2,10 @@ export const isTauri = typeof window !== 'undefined' && !!(window as any).__TAUR
 
 export function getApiBase(): string {
   if (isTauri) return 'http://localhost:8888'
-  return localStorage.getItem('proxy_url') || window.location.origin
+  const stored = localStorage.getItem('proxy_url')
+  if (stored) return stored
+  const hostname = typeof window !== 'undefined' ? window.location.hostname : ''
+  const isLocal = hostname === 'localhost' || hostname === '127.0.0.1'
+  if (isLocal) return ''  // 走 Vite proxy → Rust proxy (8888)
+  return window.location.origin
 }