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/Dockerfile

@@ -11,7 +11,7 @@ RUN npm ci
 COPY . .
 
 # 设置构建参数并构建（默认使用相对路径，由 nginx 代理）
-ARG VITE_PB_URL=/api
+ARG VITE_PB_URL=
 ENV VITE_PB_URL=$VITE_PB_URL
 
 RUN npm run build