feat: 无限画布 — infinite canvas with generation integration

Backend:
- canvases table + owner-scoped CRUD under /admin/api/canvas
  (list/create/get/save/delete, 50 docs/user, 4MiB cap, JSON validation)

Frontend:
- /canvas full-screen workspace: DOM+CSS-transform infinite pan/zoom
  (GPU-composited world layer, viewport culling, rAF-batched pointer moves)
- nodes: image (thumb display, natural-size correction), note
  (uncontrolled contenteditable), generation placeholder with edges
- generate on canvas: prompt from panel or selected notes, selected image
  nodes as references, results auto-placed + bezier edges to sources
- multi-canvas management, debounced autosave + keepalive flush,
  snapshot undo/redo (60 steps), marquee/keyboard interactions

frontend/src/layouts/PublicLayout.vue

@@ -18,6 +18,7 @@ const nav = computed(() => {
   const items = [{ to: '/', label: '首页' }]
   if (isAuthed()) {
     items.push({ to: '/user', label: '画图' })
+    items.push({ to: '/canvas', label: '画布' })
     items.push({ to: '/logs', label: '图片' })
   }
   items.push({ to: '/docs', label: '文档' })