feat(server): add localized agent starter config endpoint

Add a public GET /api/ui/agent-starters endpoint returning localized
empty-state content (title, description, preview images, and starter
prompts) for the canvas Agent panel. Content is server-configurable via
AgentStarter config with built-in zh-CN/en-US fallbacks, and the route is
exempt from authentication so shared canvases render the same content.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/handler/user_context_middleware_test.go

@@ -86,6 +86,24 @@ func TestUserContextMiddlewareIgnoresSpoofedUserIDOnPublicRoute(t *testing.T) {
 	}
 }
 
+func TestUserContextMiddlewareAllowsPublicAgentStarterConfig(t *testing.T) {
+	req := httptest.NewRequest(http.MethodGet, "/api/ui/agent-starters?locale=en-US", nil)
+	rec := httptest.NewRecorder()
+
+	called := false
+	UserContextMiddleware(fakeBearerAuthenticator{})(func(w http.ResponseWriter, _ *http.Request) {
+		called = true
+		w.WriteHeader(http.StatusNoContent)
+	})(rec, req)
+
+	if !called {
+		t.Fatal("expected public agent starter config route to continue")
+	}
+	if rec.Code != http.StatusNoContent {
+		t.Fatalf("expected status %d, got %d", http.StatusNoContent, rec.Code)
+	}
+}
+
 func TestUserContextMiddlewareAuthorizesAnonymousCanvasShare(t *testing.T) {
 	ownerID := "d1d2fbbb-97bb-4406-8d0b-ec814cc65092"
 	authorizer := fakeShareAuthorizer{access: sharingmodule.Access{