fix: 拍照提问摄像头画面倒置、下拉菜单换成前后切换按钮

真机拍照实测画面上下颠倒（Mac mini 的苹果标志倒着）。CameraCapture.vue
的 <video> 加 transform: rotate(180deg) 转正预览；capture() 画到 canvas
时同步做相同的 180° 旋转（translate 到右下角再 rotate(Math.PI)），
不然预览是正的、实际拍下来的照片还是倒的。

摄像头选择从下拉菜单换成一个悬浮在预览右上角的圆形切换按钮
（switchCamera()，在设备列表里循环切换，只有 >1 个摄像头时才显示）。
默认摄像头沿用之前"取设备列表第一个"的逻辑不变——真机实测这个正好是
朝外拍摄的那颗（另一颗是朝向操作者的），语义上更接近用户要的默认
"后置摄像头"，暂不改动。

真机验证：预览转正、切换按钮能真实切到另一颗摄像头且画面同样是正的、
拍照后的照片方向跟预览一致（不会出现预览正但存图倒的情况）。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/renderer/src/components/CameraCapture.vue

@@ -72,10 +72,23 @@ function capture(): void {
   canvas.height = video.videoHeight
   const ctx = canvas.getContext('2d')
   if (!ctx) return
+  // 这台设备的摄像头画面是倒置的（见下方 .cam-preview-video 的 CSS 旋转），
+  // 拍出来的照片也要同步转正，不然预览是正的、存下来的图是倒的
+  ctx.save()
+  ctx.translate(canvas.width, canvas.height)
+  ctx.rotate(Math.PI)
   ctx.drawImage(video, 0, 0, canvas.width, canvas.height)
+  ctx.restore()
   capturedDataUrl.value = canvas.toDataURL('image/jpeg', 0.85)
 }
 
+function switchCamera(): void {
+  if (devices.value.length < 2) return
+  const idx = devices.value.findIndex((d) => d.deviceId === selectedDeviceId.value)
+  const nextIdx = (idx + 1) % devices.value.length
+  switchDevice(devices.value[nextIdx].deviceId)
+}
+
 function retake(): void {
   capturedDataUrl.value = ''
 }
@@ -114,22 +127,23 @@ onUnmounted(() => stopStream())
         <div v-if="errorMessage" class="cam-error">{{ errorMessage }}</div>
 
         <div v-show="!capturedDataUrl" class="cam-preview-wrap">
-          <video ref="videoRef" class="cam-preview" autoplay muted playsinline />
+          <video ref="videoRef" class="cam-preview cam-preview-video" autoplay muted playsinline />
           <div v-if="starting" class="cam-loading">正在打开摄像头...</div>
+          <button
+            v-if="devices.length > 1"
+            class="cam-switch-btn"
+            title="切换前后摄像头"
+            @click="switchCamera"
+          >
+            <svg width="22" height="22" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.2" stroke-linecap="round" stroke-linejoin="round">
+              <polyline points="17 1 21 5 17 9"/><path d="M3 11V9a4 4 0 0 1 4-4h14"/><polyline points="7 23 3 19 7 15"/><path d="M21 13v2a4 4 0 0 1-4 4H3"/>
+            </svg>
+          </button>
         </div>
 
         <img v-if="capturedDataUrl" :src="capturedDataUrl" class="cam-preview" alt="拍摄预览" />
 
         <canvas ref="canvasRef" style="display:none" />
-
-        <div v-if="devices.length > 1 && !capturedDataUrl" class="cam-device-row">
-          <label class="fl">摄像头</label>
-          <select class="s-inp" :value="selectedDeviceId" @change="switchDevice(($event.target as HTMLSelectElement).value)">
-            <option v-for="d in devices" :key="d.deviceId" :value="d.deviceId">
-              {{ d.label || `摄像头 ${d.deviceId.slice(0, 6)}` }}
-            </option>
-          </select>
-        </div>
       </div>
 
       <template #footer>
@@ -154,17 +168,25 @@ onUnmounted(() => stopStream())
   width: 100%; max-height: 60vh; object-fit: contain;
   background: #000; border-radius: var(--radius-sm);
 }
+/* 这台设备的摄像头画面是倒置的，转正预览；capture() 里对拍照结果做了同样的旋转 */
+.cam-preview-video { transform: rotate(180deg); }
 .cam-loading {
   position: absolute; inset: 0; display: flex;
   align-items: center; justify-content: center;
   color: #fff; font-size: 15px; background: rgba(0,0,0,.35);
   border-radius: var(--radius-sm);
 }
+.cam-switch-btn {
+  position: absolute; top: 14px; right: 14px;
+  width: 44px; height: 44px; border-radius: 50%;
+  border: none; background: rgba(0,0,0,.45); color: #fff;
+  display: flex; align-items: center; justify-content: center;
+  cursor: pointer;
+}
+.cam-switch-btn:hover { background: rgba(0,0,0,.65); }
 .cam-error {
   color: var(--red); background: #FBEAE8;
   border-radius: var(--radius-sm); padding: 12px 14px; font-size: 15px;
 }
-.cam-device-row { display: flex; align-items: center; gap: 12px; }
-.cam-device-row .fl { flex-shrink: 0; font-size: 15px; color: var(--ink-2); }
 .cam-actions { display: flex; justify-content: flex-end; gap: 12px; }
 </style>