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/config/config.go

@@ -15,9 +15,28 @@ type Config struct {
 	Auth          AuthConfig
 	Sharing       SharingConfig
 	Agent         AgentConfig
+	AgentStarter  AgentStarterConfig
 	ObjectStorage ObjectStorageConfig
 }
 
+type AgentStarterConfig struct {
+	PreviewImages []string `json:",optional"`
+	ZhCN          AgentStarterLocaleConfig
+	EnUS          AgentStarterLocaleConfig
+}
+
+type AgentStarterLocaleConfig struct {
+	Title       string                   `json:",optional"`
+	Description string                   `json:",optional"`
+	Starters    []AgentStarterItemConfig `json:",optional"`
+}
+
+type AgentStarterItemConfig struct {
+	ID     string `json:",optional"`
+	Label  string `json:",optional"`
+	Prompt string `json:",optional"`
+}
+
 type SharingConfig struct {
 	EncryptionSecret    string `json:",optional"`
 	EncryptionSecretEnv string `json:",default=SHARING_ENCRYPTION_SECRET"`