feat: add API key validation flow for AI provider config

- Add POST /api/providers/validate backend endpoint that sends a test
  request to the provider's chat/completions API to verify the key
- Add validateProvider to frontend API client
- Redesign PanelProviders: show token input inline with Validate button,
  display valid/invalid badge after validation, Save only appears after
  successful validation
- Add i18n keys (EN/FR) for validation flow

💾 Generated with Crush

Assisted-by: GLM-5-Turbo via Crush <crush@charm.land>

web/src/api/client.js

@@ -28,6 +28,7 @@ const api = {
   savePreferences: (prefs) => request('/preferences', { method: 'PUT', body: JSON.stringify(prefs) }),
   saveProfile: (profile) => request('/config/profile', { method: 'PUT', body: JSON.stringify(profile) }),
   saveProvider: (provider) => request('/config/provider', { method: 'PUT', body: JSON.stringify(provider) }),
+  validateProvider: (provider) => request('/providers/validate', { method: 'POST', body: JSON.stringify(provider) }),
   runUpdate: (tool) => request('/update/run', { method: 'POST', body: JSON.stringify({ tool: tool || '' }) }),
   runCommand: (command, cwd) => request('/terminal', { method: 'POST', body: JSON.stringify({ command, cwd }) }),
   getTerminalSessions: () => request('/terminal/sessions'),