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>

internal/api/server.go

@@ -46,6 +46,7 @@ func (s *Server) routes() {
 	s.mux.HandleFunc("/api/mcp/configure", s.handleMCPConfigure)
 	s.mux.HandleFunc("/api/config/profile", s.handleSaveProfile)
 	s.mux.HandleFunc("/api/config/provider", s.handleSaveProvider)
+	s.mux.HandleFunc("/api/providers/validate", s.handleValidateProvider)
 	s.mux.HandleFunc("/api/update/run", s.handleRunUpdate)
 	s.mux.HandleFunc("/api/chat", s.handleChat)
 	s.mux.HandleFunc("/api/chat/history", s.handleChatHistory)