fix(public-todo): 弹窗三段式布局修复滚动与圆角，拆分两级放大

- 弹窗改为 flex 三段式：头部固定 / 内容滚动 / 底栏固定
  保存按钮常驻底部可见，外层 overflow:hidden 保住 20px 圆角
- 滚动条收进 modal-body，细圆角样式，不再贯穿整个弹窗
- 放大拆两级：弹窗头部 ⤢ 放大整个弹窗（加宽到 880px）；
  编辑器工具栏 ⤢ 仅编辑器最大化（980px x 92vh 浮层）
- Esc 优先还原编辑器最大化，再按一次才关闭弹窗

public/todo-public.html

@@ -201,9 +201,16 @@
   }
   .rte-body:empty::before { content: attr(data-ph); color: var(--text-muted); pointer-events: none; }
 
-  /* 放大书写面板：编辑弹窗右上角按钮切换 */
+  /* 弹窗级放大：弹窗头部 ⤢ 按钮切换，加宽整个弹窗 */
   .modal.lg { max-width: 880px; }
-  .modal.lg .rte-body { min-height: 48vh; max-height: none; }
+  /* 编辑器级最大化：工具栏 ⤢ 按钮切换，编辑器浮层铺满视口 */
+  .rte.max {
+    position: fixed; z-index: 80; top: 4vh; left: 50%; transform: translateX(-50%);
+    width: min(980px, 94vw); height: 92vh; display: flex; flex-direction: column;
+    background: #fff; border-color: transparent; border-radius: 16px;
+    box-shadow: 0 24px 64px rgba(15, 23, 42, .4);
+  }
+  .rte.max .rte-body { flex: 1; min-height: 0; max-height: none; padding: 18px 22px; font-size: 15px; }
 
   /* 富文本排版（编辑器与卡片展开共用 .fmt） */
   .fmt h2 { font-size: 17px; font-weight: 700; margin: 10px 0 5px; letter-spacing: -.2px; }
@@ -344,16 +351,31 @@
   @keyframes fadeIn { from { opacity: 0; } to { opacity: 1; } }
   .modal {
     width: 100%; max-width: 480px; background: #fff; border-radius: 20px;
-    box-shadow: var(--shadow-lg); padding: 24px; margin: auto;
-    animation: popIn .26s cubic-bezier(.16, 1, .3, 1); max-height: 92vh; overflow-y: auto;
+    box-shadow: var(--shadow-lg); margin: auto; padding: 0;
+    animation: popIn .26s cubic-bezier(.16, 1, .3, 1);
+    max-height: 88vh; display: flex; flex-direction: column; overflow: hidden;
   }
   @keyframes popIn { from { opacity: 0; transform: scale(.94) translateY(12px); } to { opacity: 1; transform: scale(1) translateY(0); } }
-  .modal h2 { font-size: 18px; font-weight: 700; margin-bottom: 18px; }
+  /* 三段式布局：头部固定 / 内容滚动 / 底栏固定，外层 overflow:hidden 保住圆角 */
+  .modal-head { display: flex; align-items: center; justify-content: space-between; gap: 12px; padding: 20px 24px 14px; flex-shrink: 0; }
+  .modal-head h2 { font-size: 18px; font-weight: 700; margin: 0; }
+  .modal-body { padding: 2px 24px 6px; overflow-y: auto; flex: 1; min-height: 0; }
+  .modal-body::-webkit-scrollbar, .rte-body::-webkit-scrollbar { width: 6px; }
+  .modal-body::-webkit-scrollbar-thumb, .rte-body::-webkit-scrollbar-thumb { background: #d5dbea; border-radius: 99px; }
+  .modal-body::-webkit-scrollbar-thumb:hover, .rte-body::-webkit-scrollbar-thumb:hover { background: #b6c0d8; }
+  .modal-body::-webkit-scrollbar-track, .rte-body::-webkit-scrollbar-track { background: transparent; }
+  .modal-foot { display: flex; gap: 10px; justify-content: flex-end; padding: 14px 24px; flex-shrink: 0; border-top: 1px solid var(--line); background: #fff; }
+  .mzoom {
+    width: 30px; height: 30px; border: none; background: #f1f4fa; border-radius: 8px;
+    color: var(--text-sub); cursor: pointer; font-size: 14px; flex-shrink: 0;
+    display: inline-flex; align-items: center; justify-content: center;
+    transition: background .14s, color .14s;
+  }
+  .mzoom:hover { background: rgba(99, 102, 241, .13); color: var(--indigo-dark); }
   .field { margin-bottom: 15px; }
   .field label { display: block; font-size: 13px; font-weight: 600; color: var(--text-sub); margin-bottom: 7px; }
   .field textarea { resize: vertical; min-height: 76px; }
   .field-row { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; }
-  .modal-foot { display: flex; gap: 10px; justify-content: flex-end; margin-top: 22px; }
 
   /* ===== 危险操作确认弹窗（替代浏览器原生 confirm） ===== */
   .confirm { max-width: 372px; padding: 26px 24px 20px; text-align: center; }
@@ -905,10 +927,9 @@
       var btn = e.target.closest('.rte-btn')
       if (!btn) return
       if (btn.id === 'rteExpand') {
-        var modal = root.closest('.modal')
-        var lg = modal.classList.toggle('lg')
-        btn.innerHTML = lg ? '⤡ 收起' : '⤢ 放大'
-        btn.title = lg ? '恢复默认面板' : '放大书写面板'
+        var maxed = root.classList.toggle('max')
+        btn.innerHTML = maxed ? '⤡ 还原' : '⤢ 放大'
+        btn.title = maxed ? '还原编辑器' : '编辑器最大化'
         body.focus()
         return
       }
@@ -1176,7 +1197,9 @@
     var isNew = !item
     item = item || { priority: 2, dueDate: '', remark: '', title: '' }
     var html = '<div class="mask" id="mask"><div class="modal">'
-      + '<h2>' + (isNew ? '新建待办' : '编辑待办') + '</h2>'
+      + '<div class="modal-head"><h2>' + (isNew ? '新建待办' : '编辑待办') + '</h2>'
+      + '<button type="button" class="mzoom" id="mZoom" title="放大弹窗">⤢</button></div>'
+      + '<div class="modal-body">'
       + '<div class="field"><label>标题</label>'
       + '<input type="text" id="mTitle" maxlength="200" value="' + esc(item.title) + '" placeholder="要做什么？"></div>'
       + '<div class="field-row">'
@@ -1187,6 +1210,7 @@
       + '<div class="rte">' + RTE_BAR
       + '<div class="rte-body fmt" id="mRemark" contenteditable="true" data-ph="补充说明，支持加粗 / 列表 / 待办项 / 代码块…"></div>'
       + '</div></div>'
+      + '</div>'
       + '<div class="modal-foot">'
       + '<button class="btn btn-ghost" id="mCancel">取消</button>'
       + '<button class="btn btn-primary" id="mSave">保存</button>'
@@ -1199,11 +1223,30 @@
     rteBody.innerHTML = sanitizeHtml(item.remark || '')
     initEditor(rteBody.closest('.rte'))
     var mask = $('mask')
+    var modal = mask.querySelector('.modal')
     var mTitle = $('mTitle')
     setTimeout(function () { mTitle.focus() }, 60)
 
+    // 弹窗头部 ⤢：放大整个弹窗（加宽）
+    $('mZoom').addEventListener('click', function () {
+      var lg = modal.classList.toggle('lg')
+      this.innerHTML = lg ? '⤡' : '⤢'
+      this.title = lg ? '还原弹窗' : '放大弹窗'
+    })
+
     function close() { elModalRoot.innerHTML = '' }
-    function onKey(e) { if (e.key === 'Escape') close() }
+    function onKey(e) {
+      if (e.key !== 'Escape') return
+      // 编辑器最大化时 Esc 优先还原编辑器，再按一次才关弹窗
+      var maxed = modal.querySelector('.rte.max')
+      if (maxed) {
+        maxed.classList.remove('max')
+        var expBtn = maxed.querySelector('#rteExpand')
+        if (expBtn) { expBtn.innerHTML = '⤢ 放大'; expBtn.title = '编辑器最大化' }
+        return
+      }
+      close()
+    }
 
     document.addEventListener('keydown', onKey)
     mask.addEventListener('click', function (e) { if (e.target === mask) { close(); document.removeEventListener('keydown', onKey) } })