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

lib/i18n/dictionaries/en.json

@@ -107,7 +107,13 @@
         "diagramActions": "Diagram Actions",
         "diagramActionsDescription": "Manage diagram history and exports",
         "history": "History",
-        "download": "Download"
+        "download": "Download",
+        "proxy": "Proxy Settings",
+        "proxyDescription": "Configure HTTP/HTTPS proxy for API requests (Desktop only)",
+        "httpProxy": "HTTP Proxy",
+        "httpsProxy": "HTTPS Proxy",
+        "applyProxy": "Apply",
+        "proxyApplied": "Proxy settings applied"
     },
     "save": {
         "title": "Save Diagram",