feat: add product selection step before meal planning

Inserts a new PlanProductsSheet as step 1 of the planning flow.
Users see their current products as a multi-select checklist (all
selected by default) before choosing the planning mode and dates.

- Empty state explains the benefit and offers "Add products" CTA
  while always allowing "Plan without products" to skip
- Selected product IDs flow through PlanMenuSheet →
  PlanDatePickerSheet → MenuService.generateForDates → backend
- Backend: added ProductIDs field to generate-menu request body;
  uses ListForPromptByIDs when set, ListForPrompt otherwise
- Backend: added Repository.ListForPromptByIDs (filtered SQL query)
- All 12 ARB locale files updated with planProducts* keys

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/features/menu/menu_service.dart

@@ -27,15 +27,19 @@ class MenuService {
   }
 
   /// Generates meals for specific [dates] (YYYY-MM-DD) and [mealTypes].
+  /// When [productIds] is non-empty, only those products are passed to AI.
   /// Returns the updated MenuPlan for each affected week.
   Future<List<MenuPlan>> generateForDates({
     required List<String> dates,
     required List<String> mealTypes,
+    List<String> productIds = const [],
   }) async {
-    final data = await _client.post('/ai/generate-menu', data: {
+    final body = <String, dynamic>{
       'dates': dates,
       'meal_types': mealTypes,
-    });
+    };
+    if (productIds.isNotEmpty) body['product_ids'] = productIds;
+    final data = await _client.post('/ai/generate-menu', data: body);
     final plans = data['plans'] as List<dynamic>;
     return plans
         .map((planJson) => MenuPlan.fromJson(planJson as Map<String, dynamic>))