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/plan_menu_sheet.dart

@@ -5,11 +5,17 @@ import 'package:food_ai/l10n/app_localizations.dart';
 enum PlanMode { singleMeal, singleDay, days, week }
 
 /// Bottom sheet that lets the user choose a planning horizon.
-/// Closes itself and calls [onModeSelected] with the chosen mode.
+/// Closes itself and calls [onModeSelected] with the chosen mode and the
+/// product IDs selected in the previous step (may be empty).
 class PlanMenuSheet extends StatelessWidget {
-  const PlanMenuSheet({super.key, required this.onModeSelected});
+  const PlanMenuSheet({
+    super.key,
+    required this.onModeSelected,
+    required this.selectedProductIds,
+  });
 
-  final void Function(PlanMode mode) onModeSelected;
+  final void Function(PlanMode mode, List<String> productIds) onModeSelected;
+  final List<String> selectedProductIds;
 
   @override
   Widget build(BuildContext context) {
@@ -61,7 +67,7 @@ class PlanMenuSheet extends StatelessWidget {
 
   void _select(BuildContext context, PlanMode mode) {
     Navigator.pop(context);
-    onModeSelected(mode);
+    onModeSelected(mode, selectedProductIds);
   }
 }