feat(video): add video fit toggle button and update styling

- Add toggle button to switch between 'cover' and 'contain' video fit modes
- Update video background styling for simpler positioning
- Group upload and fit buttons in a flex container

style.css

@@ -15,15 +15,12 @@ html, body {
 /* --- 视频背景 --- */
 #bg-video {
     position: fixed;
-    top: 50%;
-    left: 50%;
-    min-width: 100%;
-    min-height: 100%;
-    width: auto;
-    height: auto;
-    transform: translateX(-50%) translateY(-50%);
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
     z-index: -1; /* 将视频置于最底层 */
-    object-fit: cover; /* 关键：让视频填满整个屏幕而不变形 */
+    object-fit: cover; /* 默认填满整个屏幕（裁剪），JS会切换它 */
     background-color: #1a1a1a; /* 视频加载前的背景色 */
 }
 
@@ -77,6 +74,11 @@ html, body {
     /* 这个区域是弹性填充的，所以不需要太多样式 */
 }
 
+.button-container {
+    display: flex;
+    gap: 10px; /* 按钮之间的间距 */
+}
+
 .upload-button {
     background-color: rgba(255, 255, 255, 0.2);
     border: 1px solid rgba(255, 255, 255, 0.5);