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/API.md

@@ -20,6 +20,53 @@ Base path: `/api`
 
 Returns credits, quick actions, recent projects, and inspiration items for the Moteva-style home screen.
 
+## Agent Starters
+
+`GET /api/ui/agent-starters?locale=zh-CN`
+
+Returns the localized empty-state content used by the canvas Agent panel. The endpoint is public so shared canvases can render the same content. Supported locales are `zh-CN` and `en-US`; unsupported values fall back to `zh-CN`.
+
+```json
+{
+  "title": "今天想创作什么？",
+  "description": "输入提示词开始——或使用下方的起点",
+  "previewImages": [
+    "https://static.codia.ai/public/images/f788bdcc/lg.webp",
+    "https://static.codia.ai/public/images/ec25067a/lg.webp",
+    "https://static.codia.ai/public/images/d4ccbe39/lg.webp"
+  ],
+  "starters": [
+    {
+      "id": "ecommerce",
+      "label": "电商商品图",
+      "prompt": "Create a premium ecommerce product image for a portable espresso machine, with realistic lighting, clean composition, benefit callouts, and marketplace-ready visual hierarchy"
+    }
+  ]
+}
+```
+
+The full block can be overridden through `AgentStarter` service configuration. `PreviewImages` accepts up to three URLs; each locale owns its ordered `Starters` list. Invalid or empty configured values fall back to the built-in content, preserving the frontend contract until an operations content store replaces the configuration source.
+
+```yaml
+AgentStarter:
+  PreviewImages:
+    - https://cdn.example.com/agent-entry-1.webp
+  ZhCN:
+    Title: 今天想创作什么？
+    Description: 输入提示词开始——或使用下方的起点
+    Starters:
+      - ID: campaign
+        Label: 运营活动
+        Prompt: Create a campaign visual for the current promotion
+  EnUS:
+    Title: What would you like to create today?
+    Description: Enter a prompt to begin, or use one of the starting points below.
+    Starters:
+      - ID: campaign
+        Label: Campaign
+        Prompt: Create a campaign visual for the current promotion
+```
+
 ## Auth
 
 Global mode is enabled in `etc/config.yaml`. Email verification codes are stored through the cache abstraction for 10 minutes; with `Cache.Driver: redis`, the code only lives in Redis until TTL or successful consumption.