fix(canvas): E2E-hardened interaction fixes

- Teleport canvas to body: layout <main> z-10 stacking context let the
  fixed site header (z-40) cover the canvas toolbar
- defer setPointerCapture until the 3px drag threshold: immediate capture
  retargets click/dblclick to the stage, which silently broke note editing
- commitNote no longer early-returns on the editing flag (event-ordering
  could drop typed text); sync node.text back to the non-controlled
  contenteditable on undo/redo restores
- contenteditable only while editing: single click used to focus and allow
  raw typing outside edit mode, swallowing Delete-key handling

frontend/src/views/CanvasView.vue

@@ -551,7 +551,10 @@ const saveLabel = computed(() => ({
 </script>
 
 <template>
-  <div class="cv-page">
+  <!-- 传送到 body:布局 <main> 自带 relative z-10 层叠上下文,留在里面会被
+       站点固定顶栏(z-40)盖住,画布工具栏无法点击。 -->
+  <Teleport to="body">
+    <div class="cv-page">
     <!-- 顶栏 -->
     <div class="cv-top">
       <div class="cv-top__left">
@@ -675,7 +678,8 @@ const saveLabel = computed(() => ({
         </div>
       </div>
     </div>
-  </div>
+    </div>
+  </Teleport>
 </template>
 
 <style scoped>
@@ -719,7 +723,8 @@ const saveLabel = computed(() => ({
 .cv-page {
   position: fixed;
   inset: 0;
-  z-index: 40;
+  /* 站点顶栏是 z-40;画布全屏接管时必须盖住它 */
+  z-index: 50;
   display: flex;
   flex-direction: column;
   background: #0e1015;