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>

tool/demo_server/docker-compose.yml

@@ -0,0 +1,16 @@
+# A throwaway Nextcloud that only ever holds fake data - the backend the Play
+# Store screenshots are taken against (see tool/screenshots.sh). Nothing is
+# persisted: `docker compose down -v` wipes it completely.
+services:
+  nextcloud:
+    image: ${NEXTCLOUD_IMAGE:-nextcloud:apache}
+    ports:
+      - "${DEMO_PORT:-80}:80"
+    environment:
+      # SQLite keeps this to a single container.
+      SQLITE_DATABASE: nextcloud
+      NEXTCLOUD_ADMIN_USER: admin
+      NEXTCLOUD_ADMIN_PASSWORD: demo-admin-password
+      # `localhost` is how the Android emulator reaches this server (via
+      # `adb reverse`); DEMO_HOST is the optional friendlier hostname.
+      NEXTCLOUD_TRUSTED_DOMAINS: "localhost 127.0.0.1 10.0.2.2 ${DEMO_HOST:-demo.example.com}"