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/get_agent_starter_config_handler.go

@@ -0,0 +1,31 @@
+// Code scaffolded by goctl. Safe to edit.
+// goctl 1.10.1
+
+package handler
+
+import (
+	"net/http"
+
+	"github.com/zeromicro/go-zero/rest/httpx"
+	"img_infinite_canvas/internal/logic"
+	"img_infinite_canvas/internal/svc"
+	"img_infinite_canvas/internal/types"
+)
+
+func getAgentStarterConfigHandler(svcCtx *svc.ServiceContext) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		var req types.GetAgentStarterConfigRequest
+		if err := httpx.Parse(r, &req); err != nil {
+			httpx.ErrorCtx(r.Context(), w, err)
+			return
+		}
+
+		l := logic.NewGetAgentStarterConfigLogic(r.Context(), svcCtx)
+		resp, err := l.GetAgentStarterConfig(&req)
+		if err != nil {
+			httpx.ErrorCtx(r.Context(), w, err)
+		} else {
+			httpx.OkJsonCtx(r.Context(), w, resp)
+		}
+	}
+}