fix: v2.3 组合排列 3D 重叠修复

- CombineCanvas 新增 autoArrange: 单元按 X 序紧贴排布+底边对齐
- 拖动松手重叠检测: 与其他单元相交时自动重新排列
  (消除 3D 玻璃面穿插与 z-fighting)
- 工具栏新增自动排列按钮
- 移除 CombineCanvas 孤儿 @drop(归父容器统一处理)

frontend/src/components/CombineCanvas.vue

@@ -305,9 +305,15 @@ function onMove(e: MouseEvent) {
 
 function onUp() {
   if (dragUnit) {
-    if (dragUnit.moved) emit('change')
+    const uid = dragUnit.uid
+    const moved = dragUnit.moved
     guides = { v: [], h: [] }
     dragUnit = null
+    if (moved && uid !== null && unitOverlaps(uid)) {
+      // 拖动后与其他单元重叠 → 自动重新排列消除重叠
+      autoArrange()
+      return
+    }
     render()
     return
   }
@@ -362,6 +368,40 @@ function nudgeSelected(dx: number, dy: number) {
   emit('change')
 }
 
+function rectsIntersect(a: BBox, b: BBox): boolean {
+  return a.x0 < b.x1 - 1 && b.x0 < a.x1 - 1 && a.y0 < b.y1 - 1 && b.y0 < a.y1 - 1
+}
+
+/** 单元是否与其它单元重叠 */
+function unitOverlaps(uid: number): boolean {
+  const bb = unitBBox(uid)
+  if (!bb) return false
+  return uids().filter(u => u !== uid).some(u => {
+    const o = unitBBox(u)
+    return o ? rectsIntersect(bb, o) : false
+  })
+}
+
+/** 自动排列: 全部单元按 X 序紧贴排布、底边对齐（消除重叠） */
+function autoArrange() {
+  const ids = uids().filter(u => unitBBox(u)).sort((a, b) =>
+    (unitBBox(a)!.x0) - (unitBBox(b)!.x0))
+  if (!ids.length) return
+  let bottom = Infinity
+  for (const id of ids) {
+    const bb = unitBBox(id)!
+    bottom = Math.min(bottom, bb.y0)
+  }
+  let x = ids.length ? unitBBox(ids[0])!.x0 : 0
+  for (const id of ids) {
+    const bb = unitBBox(id)!
+    translateUid(id, x - bb.x0, bottom - bb.y0)
+    x += bb.x1 - bb.x0
+  }
+  refresh()
+  emit('change')
+}
+
 function exportDrawing(): string | null {
   if (!props.shapes.length) return null
   return JSON.stringify({
@@ -369,7 +409,7 @@ function exportDrawing(): string | null {
     epi: [], sbf: [], wlc: '', wall: [], scale: 0, note: [],
   })
 }
-defineExpose({ refresh, modelAt, exportDrawing, rotateUnit, mirrorUnit, resizeSelected, nudgeSelected })
+defineExpose({ refresh, modelAt, exportDrawing, rotateUnit, mirrorUnit, resizeSelected, nudgeSelected, autoArrange })
 
 onMounted(() => {
   watch(() => props.shapes, () => { selectedUid.value = null; rebuild() }, { immediate: true })