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/lib/likes/like_button.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../api/endpoints/likes.dart';
+import '../shared/widgets/lucide_heart.dart';
 import '../theme/theme_extension.dart';
 import 'likes_provider.dart';
 
@@ -25,8 +26,8 @@ class LikeButton extends ConsumerWidget {
       orElse: () => false,
     );
     return IconButton(
-      icon: Icon(
-        liked ? Icons.favorite : Icons.favorite_border,
+      icon: LucideHeart(
+        filled: liked,
         color: liked ? fs.accent : fs.ash,
         size: size,
       ),