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/shared/widgets/lucide_heart.dart

@@ -0,0 +1,43 @@
+import 'package:flutter/widgets.dart';
+import 'package:flutter_svg/flutter_svg.dart';
+
+/// The Lucide "heart" silhouette rendered as either an outline (stroke)
+/// or a solid fill. Lucide ships only an outline heart, so the liked
+/// state fills the same authoritative Lucide path (verified verbatim
+/// from lucide-icons/lucide) — keeping both states visually Lucide
+/// rather than introducing a Material filled heart. Used by LikeButton
+/// (and, re-derived to a VectorDrawable, by the media notification).
+class LucideHeart extends StatelessWidget {
+  const LucideHeart({
+    required this.filled,
+    required this.color,
+    this.size = 22,
+    super.key,
+  });
+
+  final bool filled;
+  final Color color;
+  final double size;
+
+  static const _path =
+      'M2 9.5a5.5 5.5 0 0 1 9.591-3.676.56.56 0 0 0 .818 0A5.49 5.49 0 0 1 '
+      '22 9.5c0 2.29-1.5 4-3 5.5l-5.492 5.313a2 2 0 0 1-3 .019L5 15c-1.5-1.5'
+      '-3-3.2-3-5.5';
+
+  @override
+  Widget build(BuildContext context) {
+    final svg = filled
+        ? '<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">'
+            '<path d="$_path" fill="currentColor"/></svg>'
+        : '<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">'
+            '<path d="$_path" fill="none" stroke="currentColor" '
+            'stroke-width="2" stroke-linecap="round" '
+            'stroke-linejoin="round"/></svg>';
+    return SvgPicture.string(
+      svg,
+      width: size,
+      height: size,
+      colorFilter: ColorFilter.mode(color, BlendMode.srcIn),
+    );
+  }
+}