Add share-to-upload, per-folder sort, and media viewer polish

- Accept Android's "Share to..." intent (ACTION_SEND/SEND_MULTIPLE) to
  upload a shared file, with a new destination-folder picker screen
- Unlink Files/Photos sort settings and make Files remember sort
  per folder (like per-folder view settings), instead of one shared
  global sort field for both tabs
- Media viewer: show the file name (marquee-scrolling if too long) in
  the top bar next to a resized back button, slide the top bar out of
  view instead of fading it, and unify icon color/contrast across the
  top bar, bottom action bar, and video transport controls
- Add a scrollbar thumb to every scrollable list in the app (all tab
  views via SyncedHeaderScaffold, search results, settings, the new
  upload folder picker, share sheet, and the Details sheet's tabs)

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

lib/views/files_view.dart

@@ -350,19 +350,21 @@ class _FilesViewState extends State<FilesView> {
               children: [
                 IconButton(
                   icon: Icon(
-                    provider.sortAscending
+                    provider.filesSortAscending
                         ? Icons.arrow_upward_rounded
                         : Icons.arrow_downward_rounded,
                     size: 20,
                   ),
                   visualDensity: VisualDensity.compact,
-                  tooltip: provider.sortAscending ? 'Ascending' : 'Descending',
-                  onPressed: provider.toggleSortOrder,
+                  tooltip: provider.filesSortAscending
+                      ? 'Ascending'
+                      : 'Descending',
+                  onPressed: provider.toggleFilesSortOrder,
                 ),
                 Expanded(
                   child: SortMenuButton(
-                    field: provider.sortField,
-                    onChanged: provider.setSortField,
+                    field: provider.filesSortField,
+                    onChanged: provider.setFilesSortField,
                   ),
                 ),
                 ToggleIconButton(