feat(favicon): add favicon handling and serve functionality

* Implement favicon serving endpoint
* Add favicon.ico and favicon-source.png files

internal/app/app.go

@@ -113,6 +113,7 @@ func routes(logger *slog.Logger, cfg *config.Config, db *store.DB, provider ai.P
 
 	mcpHandler := mcpserver.New(cfg.MCP, toolSet)
 	mux.Handle(cfg.MCP.Path, auth.Middleware(cfg.Auth, keyring, logger)(mcpHandler))
+	mux.HandleFunc("/favicon.ico", serveFavicon)
 
 	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)