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/viewer/viewer_icon_button.dart

@@ -0,0 +1,48 @@
+import 'package:flutter/material.dart';
+import '../../theme/design_tokens.dart';
+
+/// Icon-only action button for the media viewer's floating chrome (top bar,
+/// bottom action bar, video transport controls) - a fixed 22px icon in a
+/// round hit box, colored from [NooColors] rather than Material's
+/// [ColorScheme]. Kept as one small widget (matching the pre-rework
+/// `_ActionIconButton` it replaces - see `styling.md`) so every floating
+/// control in this screen looks the same instead of drifting apart across
+/// the three call sites that need one.
+class ViewerIconButton extends StatelessWidget {
+  final IconData icon;
+  final String tooltip;
+  final VoidCallback? onTap;
+
+  /// Overrides the default `fg-1` (e.g. `danger` for delete). Ignored while
+  /// [onTap] is null - a disabled button always dims to the same faded fg-1
+  /// regardless of its normal color.
+  final Color? color;
+
+  const ViewerIconButton({
+    super.key,
+    required this.icon,
+    required this.tooltip,
+    required this.onTap,
+    this.color,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final colors = context.nooColors;
+    final fg = onTap == null
+        ? colors.fg1.withValues(alpha: 0.4)
+        : (color ?? colors.fg1);
+
+    return Tooltip(
+      message: tooltip,
+      child: InkWell(
+        onTap: onTap,
+        customBorder: const CircleBorder(),
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 12),
+          child: Icon(icon, color: fg, size: 22),
+        ),
+      ),
+    );
+  }
+}