Rebuild UI on the Noo design system

Replaces the Material 3 screens with the Noo design system: warm
neutrals, one violet accent, pill controls, Schibsted Grotesk/
Instrument Sans, Lucide icons, no gradients/shadows.

- New lib/widgets/noo/ component kit (core, lists, files, media, nav,
  overlays) plus lib/theme/design_tokens.dart for the color/type/space/
  radius/motion tokens.
- Rebuilt the app shell (top/bottom bars, drawer, desktop sidebar and
  toolbar), every tab (Files/Offline, Photos, Favorites, Recent,
  Activity, Trash, Shares), Settings, the lock and login screens, the
  media viewer, search, the details/share sheets, and the share-upload
  and move/copy destination pickers.
- Added a design canvas (linked from DESIGN_SYSTEM.md) covering the
  screens the spec didn't already describe, with matching Android and
  iOS chrome; wrote up the approved recipes into DESIGN_SYSTEM.md §4.
- Removed now-dead legacy widgets (media_grid_tile, swipeable_item,
  sync_status_badge, selectable_thumbnail) and updated
  architecture.md/styling.md/standards.md to describe the new
  structure and component/testing conventions.
- Added widget tests for the noo/ component kit.

This is a UI-only rework: no provider/model/service behavior changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/widgets/profile_avatar_button.dart

@@ -3,6 +3,7 @@ import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
 import '../providers/session_controller.dart';
 import '../views/account_view.dart';
+import 'noo/core/noo_avatar.dart';
 
 /// Top-bar avatar button that opens the account screen. Replaces the old
 /// "Account" bottom-nav destination, matching how Google Drive surfaces
@@ -38,7 +39,6 @@ class ProfileAvatarButton extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final provider = context.watch<SessionController>();
-    final colorScheme = Theme.of(context).colorScheme;
     final initial = provider.username.isNotEmpty
         ? provider.username[0].toUpperCase()
         : '?';
@@ -55,18 +55,7 @@ class ProfileAvatarButton extends StatelessWidget {
               MaterialPageRoute(builder: (_) => const AccountView()),
             );
           },
-          icon: CircleAvatar(
-            radius: 17,
-            backgroundColor: colorScheme.primary,
-            child: Text(
-              initial,
-              style: TextStyle(
-                color: colorScheme.onPrimary,
-                fontWeight: FontWeight.bold,
-                fontSize: 14,
-              ),
-            ),
-          ),
+          icon: NooAvatar(initials: initial, current: true, size: 32),
         ),
       ),
     );