fix: pre-release fixes for code formatting, i18n, and test suite

- Run cargo fmt to fix Rust code formatting (lib.rs)
- Add missing i18n keys: migration.success, agents.title (zh/en/ja)
- Replace hardcoded strings "Agents" and "MCP" with t() calls in App.tsx
- Fix test mocks and assertions:
  - Add providersApi.updateTrayMenu to useSettings.test.tsx mock
  - Update SettingsPage mock path in App.test.tsx
  - Fix toast message assertion in integration/SettingsDialog.test.tsx
  - Add autoSaveSettings to SettingsDialog component test mock
  - Fix loading state test to check spinner instead of title
  - Update import button name matching for selected file state
  - Fix save button test to switch to advanced tab first
  - Remove obsolete cancel button test (button no longer exists)

Test results improved from 99 passed / 17 failed to 104 passed / 11 failed

src/i18n/locales/en.json

@@ -833,5 +833,11 @@
     "colorLabel": "Icon Color",
     "selectIcon": "Select Icon",
     "preview": "Preview"
+  },
+  "migration": {
+    "success": "Configuration migrated successfully"
+  },
+  "agents": {
+    "title": "Agents"
   }
 }

src/i18n/locales/ja.json

@@ -833,5 +833,11 @@
     "colorLabel": "アイコンカラー",
     "selectIcon": "アイコンを選択",
     "preview": "プレビュー"
+  },
+  "migration": {
+    "success": "設定の移行が完了しました"
+  },
+  "agents": {
+    "title": "エージェント"
   }
 }

src/i18n/locales/zh.json

@@ -833,5 +833,11 @@
     "colorLabel": "图标颜色",
     "selectIcon": "选择图标",
     "preview": "预览"
+  },
+  "migration": {
+    "success": "配置迁移成功"
+  },
+  "agents": {
+    "title": "智能体"
   }
 }