feat(ui): add iOS Web Share button next to download link

Adds a share button to the completed-list action row that hands the
downloaded file off to the platform share sheet via navigator.share().
On iOS Safari/Chrome this surfaces the native Save-to-Photos / Save-to-
Files / AirDrop options for videos and images, and Files / 3rd-party
app targets for audio. On platforms without Web Share support (Desktop
Firefox/Chrome/Safari) the button hides itself; the existing download
link remains the universal fallback.

Implementation notes:
- canShareDownloads() requires both navigator.share AND navigator.canShare
  (Desktop Safari has the former without the latter; we always intend
  to share a file, not a URL)
- shareDownload() fetches the file via the existing buildDownloadLink()
  helper, wraps it in a File, then runs canShare() before share() so we
  can bail out cleanly on platforms that reject the MIME type
- AbortError (user dismisses sheet) is silenced; other errors logged
- Tooltip on the button explains the iOS behaviour briefly

Refs alexta69/metube#582 — addresses the 'add to Photos.app' request
without depending on the iOS Shortcut, which has had reliability issues
(cf #763).

ui/src/app/app.html

@@ -854,6 +854,9 @@
                 @if (entry[1].filename) {
                   <a href="{{buildDownloadLink(entry[1])}}" download class="btn btn-link" [attr.aria-label]="'Download result file for ' + entry[1].title"><fa-icon [icon]="faDownload" /></a>
                 }
+                @if (entry[1].filename && canShareDownloads()) {
+                  <button type="button" class="btn btn-link" [attr.aria-label]="'Share result file for ' + entry[1].title" (click)="shareDownload(entry[1])" ngbTooltip="Share — on iOS opens the share sheet (Save to Photos, Files, AirDrop…)"><fa-icon [icon]="faShareNodes" /></button>
+                }
                 <a href="{{entry[1].url}}" target="_blank" class="btn btn-link" [attr.aria-label]="'Open source URL for ' + entry[1].title"><fa-icon [icon]="faExternalLinkAlt" /></a>
                 <button type="button" class="btn btn-link" [attr.aria-label]="'Delete completed item ' + entry[1].title" (click)="delDownload('done', entry[0])"><fa-icon [icon]="faTrashAlt" /></button>
               </div>