feat(recipes): add recipe visibility and sharing features

- Implemented functionality to set recipe visibility (public/private) with appropriate checks for user permissions.
- Added ability to share recipes with other users, including validation for existing users and permissions.
- Introduced new DTOs for setting visibility and sharing recipes.
- Updated RecipesController and RecipesService to handle new endpoints for visibility and sharing.
- Enhanced inventory preview to consider user permissions and shared recipes.
- Updated front-end to support new sharing and visibility features, including UI changes for recipe detail and admin user management.

flutter/lib/core/auth/jwt_decoder.dart

@@ -24,3 +24,22 @@ String jwtRole(String? token) {
 
 /// Returns true if the JWT token contains role == 'admin'.
 bool jwtIsAdmin(String? token) => jwtRole(token) == 'admin';
+
+/// Returns username claim from JWT token, if present.
+String? jwtUsername(String? token) {
+  if (token == null || token.isEmpty) return null;
+  final claims = decodeJwtPayload(token);
+  final value = claims['username']?.toString().trim();
+  if (value == null || value.isEmpty) return null;
+  return value;
+}
+
+/// Returns user id claim from JWT token, if present.
+int? jwtUserId(String? token) {
+  if (token == null || token.isEmpty) return null;
+  final claims = decodeJwtPayload(token);
+  final raw = claims['sub'] ?? claims['userId'] ?? claims['id'];
+  if (raw is num) return raw.toInt();
+  if (raw is String) return int.tryParse(raw);
+  return null;
+}