feat(proxy): add isFullUrl toggle for full API endpoint mode

- provider.rs/types.ts: add is_full_url field to ProviderMeta
- forwarder.rs: when isFullUrl is set, use base_url directly instead of
  appending endpoint path; also handle query passthrough and strip
  beta=true when transforming to /v1/chat/completions
- EndpointField.tsx: add Link2 icon toggle button for full URL mode
- ClaudeFormFields.tsx: pass through isFullUrl/onFullUrlChange props
- ProviderForm.tsx: manage localIsFullUrl state, persist to meta on save
- useProviderActions.ts: block switching to isFullUrl or openai_chat
  providers when proxy is not running, show warning toast
- App.tsx: pass isProxyRunning to useProviderActions
- i18n: add fullUrlEnabled/fullUrlDisabled/fullUrlHint and
  proxyRequiredForSwitch translations for zh/en/ja

src/App.tsx

@@ -244,7 +244,7 @@ function App() {
     deleteProvider,
     saveUsageScript,
     setAsDefaultModel,
-  } = useProviderActions(activeApp);
+  } = useProviderActions(activeApp, isProxyRunning);
 
   const disableOmoMutation = useDisableCurrentOmo();
   const handleDisableOmo = () => {