feat: add proxy settings to Settings dialog (Desktop only) (#537)

* feat: add proxy settings to Settings dialog (Desktop only)

Fixes #535 - Desktop app now respects HTTP/HTTPS proxy configuration.

- Add proxy-manager.ts to handle proxy config storage (JSON file in userData)
- Load proxy settings on app startup before Next.js server starts
- Add IPC handlers for get-proxy and set-proxy
- Add proxy settings UI in Settings dialog (Electron only)
- Add translations for en/zh/ja

* fix: improve proxy settings reliability and simplify UI

- Fix server restart race condition (wait for process exit before starting new server)
- Add URL validation (must include http:// or https:// prefix)
- Enable Node.js built-in proxy support (NODE_USE_ENV_PROXY=1)
- Remove "Proxy Exceptions" field (unnecessary for this app)
- Add debug logging for proxy env vars

* refactor: remove duplicate ProxyConfig interface, import from electron.d.ts

electron/preload/index.ts

@@ -21,4 +21,9 @@ contextBridge.exposeInMainWorld("electronAPI", {
     // File operations
     openFile: () => ipcRenderer.invoke("dialog-open-file"),
     saveFile: (data: string) => ipcRenderer.invoke("dialog-save-file", data),
+
+    // Proxy settings
+    getProxy: () => ipcRenderer.invoke("get-proxy"),
+    setProxy: (config: { httpProxy?: string; httpsProxy?: string }) =>
+        ipcRenderer.invoke("set-proxy", config),
 })