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/views/login_webview_view.dart

@@ -42,8 +42,17 @@ class _LoginWebViewViewState extends State<LoginWebViewView> {
             if (mounted) setState(() => _pageLoading = false);
           },
         ),
-      )
-      ..loadRequest(widget.url);
+      );
+    // Android's WebView CookieManager is a single store shared/persisted
+    // across every WebView instance in the app, not scoped to this
+    // controller - without clearing it first, "Add Account" (or logging
+    // back in as a different user on the same server) would silently land
+    // on whichever account's Nextcloud session cookie is already there
+    // instead of prompting for credentials, defeating the point of
+    // starting a fresh Login Flow v2 at all.
+    WebViewCookieManager().clearCookies().then((_) {
+      if (mounted) _controller.loadRequest(widget.url);
+    });
   }
 
   void _cancel() {