feat(canvas): manual node linking + no-model hint + render-hardening

- drag from a node's side port onto another node to create an edge
  (dashed live preview, dedupe, self-link rejected); click an edge to
  select (highlighted), Delete removes it, Ctrl+Z restores — all in
  undo history and autosave
- generate panel shows an explicit hint when the site has no image
  models instead of an empty dropdown
- render hardening: viewport-culling size now has a hard floor
  (max(stageSize, window, 1600x900)) so a delayed/degenerate
  ResizeObserver reading can never cull nodes out of the DOM; stage
  listeners attach with retry + rAF self-healing measure; pointer
  capture wrapped in try/catch for synthetic pointers

frontend/src/components/canvas/CanvasNodeView.vue

@@ -99,6 +99,8 @@ defineExpose({ startEdit, commitNote })
     :data-node-id="node.id"
     @dblclick="onSelfDblClick"
   >
+    <!-- 连线出口:拖到目标节点创建连线(舞台接管手势) -->
+    <span v-if="node.type !== 'gen'" class="cv-port" data-port="out" />
     <!-- 图片节点 -->
     <template v-if="node.type === 'image'">
       <img
@@ -241,4 +243,24 @@ defineExpose({ startEdit, commitNote })
   text-overflow: ellipsis;
   white-space: nowrap;
 }
+
+.cv-port {
+  position: absolute;
+  right: -8px;
+  top: 50%;
+  transform: translateY(-50%);
+  width: 13px;
+  height: 13px;
+  border-radius: 50%;
+  background: #7c6cff;
+  border: 2.5px solid #0e1015;
+  opacity: 0;
+  transition: opacity 0.12s;
+  cursor: crosshair;
+  z-index: 2;
+}
+.cv-node:hover .cv-port,
+.cv-node.is-selected .cv-port {
+  opacity: 1;
+}
 </style>