feat(shared): legacy presentation/ + shared design system + widgets

- lib/presentation : pages legacy (explore/network, notifications) avec BLoC
- lib/shared/design_system : UnionFlow Design System v2 (tokens, components)
  + MD3 tokens + module_colors par feature
- lib/shared/widgets : widgets transversaux (core_card, core_shimmer,
  error_widget, loading_widget, powered_by_lions_dev, etc.)
- lib/shared/constants + utils

lib/shared/widgets/powered_by_lions_dev.dart

@@ -0,0 +1,90 @@
+import 'package:flutter/material.dart';
+import 'package:url_launcher/url_launcher.dart';
+import '../design_system/unionflow_design_system.dart';
+
+/// Widget "Powered by Lions Dev" — affiche le logo Lions Dev adaptatif (dark/light)
+/// avec lien cliquable vers https://www.lions.dev
+///
+/// Usage:
+/// ```dart
+/// const PoweredByLionsDev()
+/// // ou compact:
+/// const PoweredByLionsDev(compact: true)
+/// ```
+class PoweredByLionsDev extends StatelessWidget {
+  /// Si true, affichage compact (logo plus petit, sans label "Powered by")
+  final bool compact;
+
+  /// Couleur du label "Powered by" (par défaut : couleur secondaire du thème)
+  final Color? labelColor;
+
+  /// Hauteur du logo (par défaut : 28 normal, 20 compact)
+  final double? logoHeight;
+
+  /// Force une variante (utile sur fond toujours sombre/clair comme login).
+  /// Si null, suit le thème courant.
+  final Brightness? forceBrightness;
+
+  const PoweredByLionsDev({
+    super.key,
+    this.compact = false,
+    this.labelColor,
+    this.logoHeight,
+    this.forceBrightness,
+  });
+
+  Future<void> _openLionsDev() async {
+    final uri = Uri.parse('https://www.lions.dev');
+    if (await canLaunchUrl(uri)) {
+      await launchUrl(uri, mode: LaunchMode.externalApplication);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final brightness = forceBrightness ?? Theme.of(context).brightness;
+    final isDark = brightness == Brightness.dark;
+    // Logo blanc sur fond sombre, logo noir sur fond clair
+    final logoAsset = isDark
+        ? 'assets/images/branding/lions_dev_white.png'
+        : 'assets/images/branding/lions_dev_dark.png';
+
+    final effectiveLabelColor = labelColor ??
+        (isDark ? AppColors.textSecondaryDark : AppColors.textSecondary);
+
+    final effectiveHeight = logoHeight ?? (compact ? 20.0 : 28.0);
+
+    return InkWell(
+      onTap: _openLionsDev,
+      borderRadius: BorderRadius.circular(SpacingTokens.radiusMd),
+      child: Padding(
+        padding: const EdgeInsets.symmetric(
+          horizontal: SpacingTokens.md,
+          vertical: SpacingTokens.sm,
+        ),
+        child: Row(
+          mainAxisSize: MainAxisSize.min,
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            if (!compact) ...[
+              Text(
+                'Powered by',
+                style: TextStyle(
+                  fontSize: 11,
+                  color: effectiveLabelColor,
+                  letterSpacing: 0.3,
+                ),
+              ),
+              const SizedBox(width: SpacingTokens.sm),
+            ],
+            Image.asset(
+              logoAsset,
+              height: effectiveHeight,
+              fit: BoxFit.contain,
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}