Add in-app copy/move, background downloads, and a Favorites tab

- WebDAV MOVE/COPY-backed copy/move for files and folders, with a
  destination picker (independent nav state so it doesn't disturb the
  Files tab's browsing position) and a conflict-resolution sheet
  (overwrite all / keep both / decide per item).
- Downloads now use the same foreground-service + notification approach
  as uploads, surviving app closure.
- Favorites is now its own tab (account-wide, via a WebDAV SEARCH query)
  instead of a Files-tab filter toggle, which couldn't represent
  favorited items outside the currently browsed folder correctly.
- Share sheet restyled to match the app's design language, plus a
  "share file directly" action via the OS share sheet; details sheet
  tab icons resized to match the bottom nav.

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

lib/widgets/details/details_sheet.dart

@@ -51,11 +51,24 @@ class DetailsSheet extends StatelessWidget {
       child: Column(
         children: [
           DetailsHeader(item: item),
+          // Icon size matches the bottom nav bar's own tab icons
+          // (FloatingBottomNavBar) rather than TabBar's oversized default,
+          // so a "tabs" affordance reads the same size everywhere in the
+          // app.
           const TabBar(
             tabs: [
-              Tab(icon: Icon(Icons.info_outline_rounded), text: 'Info'),
-              Tab(icon: Icon(Icons.history_rounded), text: 'Versions'),
-              Tab(icon: Icon(Icons.electric_bolt_rounded), text: 'Activity'),
+              Tab(
+                icon: Icon(Icons.info_outline_rounded, size: 22),
+                text: 'Info',
+              ),
+              Tab(
+                icon: Icon(Icons.history_rounded, size: 22),
+                text: 'Versions',
+              ),
+              Tab(
+                icon: Icon(Icons.electric_bolt_rounded, size: 22),
+                text: 'Activity',
+              ),
             ],
           ),
           Expanded(