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/preview.html

@@ -23,7 +23,7 @@
             <img src="{{.Data.File.ThumbnailURL}}" alt="">
           {{end}}
         </div>
-        <h1 id="preview-title">{{.Data.File.Name}}</h1>
+        <h1 id="preview-title" class="file-name" title="{{.Data.File.Name}}">{{.Data.File.Name}}</h1>
         <p class="download-subtitle">{{.Data.File.Size}} · {{.Data.File.ContentType}}</p>
         <a class="button button-primary button-wide" href="{{.Data.DownloadURL}}">
           <svg viewBox="0 0 24 24" role="img" focusable="false" aria-hidden="true"><path d="M12 3v12m0 0 4-4m-4 4-4-4M5 21h14" /></svg>