fix: resolve /api/api doubled path in Docker deployment

- Remove VITE_PB_URL=/api from Dockerfile (SDK auto-appends /api)
- Fix nginx proxy_pass to keep /api/ prefix for PocketBase
- Update pocketbase.ts fallback to window.location.origin
- Update PocketBase proxy target to port 8090

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/pocketbase.ts

@@ -1,7 +1,7 @@
 // src/api/pocketbase.ts
 import PocketBase from 'pocketbase'
 
-const pbUrl = import.meta.env.VITE_PB_URL || 'http://localhost:8090'
+const pbUrl = import.meta.env.VITE_PB_URL || window.location.origin
 
 export const pb = new PocketBase(pbUrl)