feat(ui): Lucide migration unit 1 — dependency + heart (#60)

Design system mandates Lucide, not Material. Foundation before the
mechanical Icons.* sweep:

- pubspec: add flutter_lucide ^1.11.0
- shared/widgets/lucide_heart.dart: LucideHeart renders the verified
  lucide-icons/lucide heart path as outline (stroke) or filled, via
  flutter_svg, tinted by color — Lucide ships no filled heart, so the
  liked state fills the same Lucide silhouette (user-chosen approach)
- like_button: use LucideHeart instead of Icons.favorite/_border
- notification drawables re-derived from the verbatim Lucide heart
  path (border = stroke, filled = fill); separators spaced for
  Android pathData

Unit 2 (mechanical Icons.* -> LucideIcons.* sweep) follows once this
is CI-green. pubspec.lock regenerated by CI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/pubspec.yaml

@@ -22,6 +22,9 @@ dependencies:
   flutter_secure_storage: ^10.1.0
   go_router: ^17.2.3
   flutter_svg: ^2.0.16
+  # Lucide icon set (design system mandates Lucide, not Material).
+  # Icons exposed as LucideIcons.<snake_case> IconData usable in Icon().
+  flutter_lucide: ^1.11.0
   google_fonts: ^8.1.0
   # 10.x conflicts with flutter_secure_storage 10.x on win32. Hold at 8.3.1
   # until either lib bumps win32 to 6.x.