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_date_picker_sheet.dart

@@ -16,6 +16,7 @@ class PlanDatePickerSheet extends ConsumerStatefulWidget {
     super.key,
     required this.mode,
     required this.defaultStart,
+    required this.selectedProductIds,
   });
 
   final PlanMode mode;
@@ -24,6 +25,10 @@ class PlanDatePickerSheet extends ConsumerStatefulWidget {
   /// planned date.
   final DateTime defaultStart;
 
+  /// Product IDs selected in the previous step. Empty list means the AI
+  /// will use all of the user's products (default behaviour).
+  final List<String> selectedProductIds;
+
   @override
   ConsumerState<PlanDatePickerSheet> createState() =>
       _PlanDatePickerSheetState();
@@ -107,6 +112,7 @@ class _PlanDatePickerSheetState extends ConsumerState<PlanDatePickerSheet> {
       await ref.read(planMenuServiceProvider).generateForDates(
             dates: dates,
             mealTypes: mealTypes,
+            productIds: widget.selectedProductIds,
           );
       if (mounted) {
         Navigator.pop(context);