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",

lib/i18n/dictionaries/ja.json

@@ -107,7 +107,13 @@
         "diagramActions": "ダイアグラム操作",
         "diagramActionsDescription": "ダイアグラムの履歴とエクスポートを管理",
         "history": "履歴",
-        "download": "ダウンロード"
+        "download": "ダウンロード",
+        "proxy": "プロキシ設定",
+        "proxyDescription": "API リクエスト用の HTTP/HTTPS プロキシを設定（デスクトップ版のみ）",
+        "httpProxy": "HTTP プロキシ",
+        "httpsProxy": "HTTPS プロキシ",
+        "applyProxy": "適用",
+        "proxyApplied": "プロキシ設定が適用されました"
     },
     "save": {
         "title": "ダイアグラムを保存",

lib/i18n/dictionaries/zh.json

@@ -107,7 +107,13 @@
         "diagramActions": "图表操作",
         "diagramActionsDescription": "管理图表历史记录和导出",
         "history": "历史记录",
-        "download": "下载"
+        "download": "下载",
+        "proxy": "代理设置",
+        "proxyDescription": "配置 API 请求的 HTTP/HTTPS 代理（仅桌面版）",
+        "httpProxy": "HTTP 代理",
+        "httpsProxy": "HTTPS 代理",
+        "applyProxy": "应用",
+        "proxyApplied": "代理设置已应用"
     },
     "save": {
         "title": "保存图表",