feat(ui): truncate long file names and auto-close context menu

- Add `.file-name` class with ellipsis truncation for long file names to prevent layout overflow.
- Apply truncation to metadata and file items in download and preview pages.
- Add `title` attributes to truncated names to show the full text on hover.
- Automatically close the file context menu when the mouse moves more than 80px away from it.

backend/templates/pages/download.html

@@ -49,7 +49,7 @@
                 <img src="{{.ThumbnailURL}}" alt="" loading="lazy">
               </a>
               <a class="file-main" href="{{.DownloadURL}}?inline=1">
-                <strong>{{.Name}}</strong>
+                <strong class="file-name" title="{{.Name}}">{{.Name}}</strong>
                 <small>{{.Size}} · {{.ContentType}}</small>
               </a>
               {{if not $.Data.Locked}}