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/routes.go

@@ -329,6 +329,11 @@ func RegisterHandlers(server *rest.Server, serverCtx *svc.ServiceContext) {
 				Path:    "/shares/:shareId",
 				Handler: resolveShareHandler(serverCtx),
 			},
+			{
+				Method:  http.MethodGet,
+				Path:    "/ui/agent-starters",
+				Handler: getAgentStarterConfigHandler(serverCtx),
+			},
 		},
 		rest.WithPrefix("/api"),
 	)