feat(settings): add 'Apply to Claude Code extension' toggle

- Apply immediately on save (write or remove primaryApiKey)
- Honor setting on provider switch (enabled: write for non-official, remove for official; disabled: no auto writes)
- Remove per-provider Claude plugin buttons from ProviderList
- Upsert primaryApiKey=any preserving other fields; respect override dir
- Add zh/en i18n for the new setting

src/i18n/locales/en.json

@@ -98,6 +98,8 @@
     "windowBehavior": "Window Behavior",
     "minimizeToTray": "Minimize to tray on close",
     "minimizeToTrayDescription": "When checked, clicking the close button will hide to system tray, otherwise the app will exit directly.",
+    "enableClaudePluginIntegration": "Apply to Claude Code extension",
+    "enableClaudePluginIntegrationDescription": "When enabled, you can use third-party providers in the VS Code Claude Code extension",
     "configFileLocation": "Configuration File Location",
     "openFolder": "Open Folder",
     "configDirectoryOverride": "Configuration Directory Override (Advanced)",