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/app_top_bar.dart

@@ -0,0 +1,64 @@
+import 'package:flutter/material.dart';
+import 'package:lucide_icons_flutter/lucide_icons.dart';
+import '../models/app_tab.dart';
+import 'create_menu.dart';
+import 'noo/nav/noo_top_bar.dart';
+import 'shell/shell_common.dart';
+
+/// The shell's mobile top bar (DESIGN_SYSTEM.md 3, "Mobile"): every regular
+/// tab now gets the same chrome (this used to only wrap Files - the other
+/// tabs built their own via `SyncedHeaderScaffold`, which the rework drops).
+/// Built on [NooTopBar]:
+///
+/// - iOS: `menu` leading, `plus` (Files only - no tab besides Files has a
+///   create/upload flow) and the account avatar trailing, then the large
+///   title, then an inline search field that opens [SearchView].
+/// - Android: `menu`, title, `search` and avatar actions. Upload has no
+///   top-bar icon here - it's the [NooFab] `MainShellView` shows instead.
+///
+/// [style] is resolved by the caller (`NooLayout.navStyle`) rather than
+/// read from context here, since [preferredSize] is a plain getter with no
+/// BuildContext - the same reason [NooTopBar] itself takes it as a field.
+class AppTopBar extends StatelessWidget implements PreferredSizeWidget {
+  final NooNavStyle style;
+  final AppTab tab;
+
+  const AppTopBar({super.key, required this.style, required this.tab});
+
+  bool get _ios => style == NooNavStyle.ios;
+
+  @override
+  Size get preferredSize => NooTopBar(
+    style: style,
+    title: tab.label,
+    // A throwaway placeholder purely to make preferredSize's "is there a
+    // search slot" check match what build() actually shows - never
+    // inserted into the tree, so its content doesn't matter.
+    search: _ios ? const SizedBox.shrink() : null,
+  ).preferredSize;
+
+  @override
+  Widget build(BuildContext context) {
+    return NooTopBar(
+      style: style,
+      title: tab.label,
+      onMenu: () => Scaffold.of(context).openDrawer(),
+      actions: [
+        if (!_ios)
+          NooTopBarButton(
+            icon: LucideIcons.search,
+            tooltip: 'Search',
+            onTap: () => openSearch(context),
+          ),
+        if (_ios && tab == AppTab.files)
+          NooTopBarButton(
+            icon: LucideIcons.plus,
+            tooltip: 'Create',
+            onTap: () => showCreateMenu(context),
+          ),
+        ShellAvatarButton(hitBox: _ios ? 44 : 48),
+      ],
+      search: _ios ? const ShellSearchLauncher(ios: true) : null,
+    );
+  }
+}