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/models/app_tab.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:material_symbols_icons/symbols.dart';
+import 'package:lucide_icons_flutter/lucide_icons.dart';
 
 /// Every destination the bottom nav bar can show. Order here is only the
 /// fallback default — actual order/visibility/default-tab are user
@@ -41,24 +41,26 @@ extension AppTabInfo on AppTab {
     }
   }
 
+  // Lucide icon mapping per DESIGN_SYSTEM.md 1.5 - the Lucide icon slug is
+  // named in a comment where it doesn't match the Dart identifier.
   IconData get icon {
     switch (this) {
       case AppTab.files:
-        return Icons.folder_rounded;
+        return LucideIcons.folder;
       case AppTab.photos:
-        return Icons.photo_library_rounded;
+        return LucideIcons.images;
       case AppTab.favorites:
-        return Icons.favorite_rounded;
+        return LucideIcons.star;
       case AppTab.activity:
-        return Icons.electric_bolt_rounded;
+        return LucideIcons.activity;
       case AppTab.trash:
-        return Icons.delete_outline_rounded;
+        return LucideIcons.trash2; // trash-2
       case AppTab.shares:
-        return Icons.groups_rounded;
+        return LucideIcons.share2; // share-2
       case AppTab.recent:
-        return Symbols.search_activity_rounded;
+        return LucideIcons.clock;
       case AppTab.offline:
-        return Icons.offline_pin_rounded;
+        return LucideIcons.hardDriveDownload; // hard-drive-download
     }
   }
 }