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/nginx.conf

@@ -14,7 +14,7 @@ server {
 
     # API 代理到局域网 PocketBase
     location /api/ {
-        proxy_pass http://192.168.1.14:8711/;
+        proxy_pass http://192.168.1.14:8090;
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection 'upgrade';