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.go

@@ -107,7 +107,8 @@ func requiresAuthenticatedUser(r *http.Request) bool {
 		strings.HasPrefix(path, "/api/shares/") ||
 		path == "/api/auth/options" ||
 		path == "/api/health" ||
-		path == "/api/dashboard" {
+		path == "/api/dashboard" ||
+		path == "/api/ui/agent-starters" {
 		return false
 	}
 	return true