feat: API docs page — tabbed webhook spec, scoring, challenges, tips

Fetches from /api/docs/webhook and renders as a styled tabbed page.
Added /docs route and DOCS nav link.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/router.ts

@@ -51,6 +51,11 @@ const routes = [
     name: 'sprites',
     component: () => import('./pages/SpritePreviewPage.vue'),
   },
+  {
+    path: '/docs',
+    name: 'docs',
+    component: () => import('./pages/DocsPage.vue'),
+  },
 ]
 
 export const router = createRouter({