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/bottom_nav_bar.dart

@@ -0,0 +1,36 @@
+import 'package:flutter/material.dart';
+import '../models/app_tab.dart';
+import 'noo/nav/noo_bottom_bar.dart';
+
+/// Adapts the app's [AppTab] model onto [NooBottomBar]'s generic
+/// [NooNavDestination] list, so `MainShellView` only deals in tabs and
+/// doesn't need to import `noo/nav` itself. [selectedIndex] may be -1 (no
+/// destination highlighted) when the shell is showing a tab opened from the
+/// drawer's "More" list that isn't one of the pinned [tabs].
+class BottomNavBar extends StatelessWidget {
+  final NooNavStyle style;
+  final List<AppTab> tabs;
+  final int selectedIndex;
+  final ValueChanged<int> onDestinationSelected;
+
+  const BottomNavBar({
+    super.key,
+    required this.style,
+    required this.tabs,
+    required this.selectedIndex,
+    required this.onDestinationSelected,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return NooBottomBar(
+      style: style,
+      destinations: [
+        for (final tab in tabs)
+          NooNavDestination(icon: tab.icon, label: tab.label),
+      ],
+      selectedIndex: selectedIndex,
+      onSelected: onDestinationSelected,
+    );
+  }
+}