feat: 确认/取消按钮跟随截图区域，显示在选择区右下角

src/components/ScreenshotOverlay.vue

@@ -8,7 +8,7 @@
   >
     <canvas ref="canvasRef" class="screenshot-canvas"></canvas>
     <div v-if="!hasSelection" class="screenshot-tip">拖动选择截图区域 · Esc取消</div>
-    <div v-else class="screenshot-actions" @mousedown.stop @mouseup.stop>
+    <div v-else class="screenshot-actions" :style="actionsStyle" @mousedown.stop @mouseup.stop>
       <button class="action-btn confirm" @click="confirmCapture">✓ 确认</button>
       <button class="action-btn cancel" @click="cancel">✕ 取消</button>
     </div>
@@ -29,6 +29,21 @@ const state = {
 const hasSelection = ref(false)
 const isCapturing = ref(false)
 
+const actionsStyle = computed(() => {
+  if (!hasSelection.value) return {}
+  const x = Math.min(state.startX, state.endX)
+  const y = Math.min(state.startY, state.endY)
+  const w = Math.abs(state.endX - state.startX)
+  const h = Math.abs(state.endY - state.startY)
+  const panelW = 160
+  const gap = 8
+  let left = x + w - panelW
+  let top = y + h + gap
+  if (left < 4) left = 4
+  if (top + 40 > window.innerHeight) top = y - 40 - gap
+  return { left: left + 'px', top: top + 'px', transform: 'none', bottom: 'auto' }
+})
+
 onMounted(() => {
   document.addEventListener('keydown', onKeyDown)
 })
@@ -172,9 +187,6 @@ function onKeyDown(e: KeyboardEvent) {
 
 .screenshot-actions {
   position: fixed;
-  bottom: 40px;
-  left: 50%;
-  transform: translateX(-50%);
   display: flex;
   gap: 10px;
   pointer-events: auto;