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/img_infinite_canvas.api

@@ -307,6 +307,23 @@ type DashboardResponse {
 	Inspiration  []InspirationItem `json:"inspiration"`
 }
 
+type GetAgentStarterConfigRequest {
+	Locale string `form:"locale,optional"`
+}
+
+type AgentStarterItem {
+	Id     string `json:"id"`
+	Label  string `json:"label"`
+	Prompt string `json:"prompt"`
+}
+
+type AgentStarterConfigResponse {
+	Title         string             `json:"title"`
+	Description   string             `json:"description"`
+	PreviewImages []string           `json:"previewImages"`
+	Starters      []AgentStarterItem `json:"starters"`
+}
+
 type ProjectListResponse {
 	Projects   []ProjectSummary `json:"projects"`
 	NextOffset int64            `json:"nextOffset"`
@@ -1007,6 +1024,9 @@ service img_infinite_canvas-api {
 	@handler dashboard
 	get /dashboard returns (DashboardResponse)
 
+	@handler getAgentStarterConfig
+	get /ui/agent-starters (GetAgentStarterConfigRequest) returns (AgentStarterConfigResponse)
+
 	@handler getAuthOptions
 	get /auth/options returns (AuthOptionsResponse)