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/README.md

@@ -5,6 +5,7 @@ go-zero backend for the Moteva-style infinite canvas MVP.
 ## Features
 
 - Dashboard data for the Moteva home page
+- Localized, server-configurable Agent starter content for the empty canvas chat
 - Project creation from prompt
 - User-owned Brand Kits with optional per-canvas binding and server-authoritative creative context
 - Project title rename