Add GET_CONTENT picker support, fix account/login bugs, restyle selection UI

- Let Noo be picked as a source (Files + Photos) by other apps' native
  file/photo pickers via Android's GET_CONTENT intent, mirroring the
  existing "Share to Noo" plumbing: MainActivity.kt/PickIntentService/
  PickRequest, ServerProvider.pickRequest/confirmPick/cancelPick, and
  pick-mode tap handling in FilesView/PhotosView. While picking, the
  bottom nav and MoreTabsButton are restricted to Files/Photos only.
- Fix two account bugs: testConnection() failures no longer delete a
  still-valid stored app password (only an actual 401 does), and
  switchAccount() now retries a nominally-active-but-logged-out account
  instead of no-op'ing - both were silently locking users out of "Continue
  as" with no way back in except removing and re-adding the account
  (now exposed via login_view.dart's inline remove button).
- Fix the login WebView: clear cookies before every load so "Add Account"
  can't silently reuse an existing session, and push it as a
  non-animated route to avoid a known WebView-as-PlatformView black-screen-
  on-pop issue under Impeller.
- Redesign the Files/Photos selection toolbar to take over the top app bar
  entirely (SyncedHeaderScaffold.selectionBar) instead of sitting as a
  second bar under the controls row, with a horizontally-scrollable,
  uniformly-colored actions row and the same one-shot scroll-hint nudge
  the controls row uses.
- Redesign ShareUploadView to share the same SyncedHeaderScaffold chrome
  as every tab, with the uploading-file summary (marqueed via the new
  shared MarqueeTitle widget) and upload button grouped into one
  rounded/elevated bottom sheet.
- Restyle ShareSheet to open on the same DetailsHeader every other
  per-item sheet uses, and fix a couple of small design-language drifts
  (hardcoded text style, a non-rounded icon).
- Settings' account card: icon buttons with tooltips instead of text
  buttons for refresh/logout/remove, divider removed.

lib/widgets/details/details_sheet.dart

@@ -73,13 +73,25 @@ class DetailsSheet extends StatelessWidget {
   }
 }
 
-/// Icon/name/size/date row shared by [DetailsSheet] and the media viewer's
+/// Icon/name/size/date row shared by [DetailsSheet], the media viewer's
 /// collapsed "peek" state (which shows just this, before the user drags the
-/// sheet open far enough to reveal the tabs).
+/// sheet open far enough to reveal the tabs), and [ShareSheet]'s own title
+/// (so every per-item bottom sheet opens on the same header instead of
+/// [ShareSheet] alone using a bare title `Text`).
 class DetailsHeader extends StatelessWidget {
   final NextcloudItem item;
 
-  const DetailsHeader({super.key, required this.item});
+  /// Defaults to this widget's own standalone inset ([DetailsSheet]); a
+  /// caller whose surrounding scroll view already applies horizontal
+  /// padding (e.g. [ShareSheet]'s `ListView`) should pass a padding with
+  /// zero left/right to avoid doubling it up.
+  final EdgeInsetsGeometry padding;
+
+  const DetailsHeader({
+    super.key,
+    required this.item,
+    this.padding = const EdgeInsets.fromLTRB(20, 12, 20, 8),
+  });
 
   @override
   Widget build(BuildContext context) {
@@ -87,7 +99,7 @@ class DetailsHeader extends StatelessWidget {
     final colorScheme = theme.colorScheme;
 
     return Padding(
-      padding: const EdgeInsets.fromLTRB(20, 12, 20, 8),
+      padding: padding,
       child: Row(
         children: [
           Container(