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/types/types.go

@@ -77,6 +77,19 @@ type AgentPreferToolCategories struct {
 	Image  []string `json:"IMAGE,optional"`
 }
 
+type AgentStarterConfigResponse struct {
+	Title         string             `json:"title"`
+	Description   string             `json:"description"`
+	PreviewImages []string           `json:"previewImages"`
+	Starters      []AgentStarterItem `json:"starters"`
+}
+
+type AgentStarterItem struct {
+	Id     string `json:"id"`
+	Label  string `json:"label"`
+	Prompt string `json:"prompt"`
+}
+
 type AgentThreadListRequest struct {
 	ProjectId string `json:"projectId"`
 	Page      int64  `json:"page,optional"`
@@ -545,6 +558,10 @@ type GeneratorTaskSubmitResponse struct {
 	Data    GeneratorTaskSubmitData `json:"data"`
 }
 
+type GetAgentStarterConfigRequest struct {
+	Locale string `form:"locale,optional"`
+}
+
 type GlobalEmailLoginRequest struct {
 	Email string `json:"email"`
 	Code  string `json:"code"`