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_provider.dart

@@ -192,11 +192,13 @@ class PlanMenuService {
   Future<void> generateForDates({
     required List<String> dates,
     required List<String> mealTypes,
+    List<String> productIds = const [],
   }) async {
     final menuService = _ref.read(menuServiceProvider);
     final plans = await menuService.generateForDates(
       dates: dates,
       mealTypes: mealTypes,
+      productIds: productIds,
     );
     for (final plan in plans) {
       _ref.invalidate(menuProvider(isoWeekString(DateTime.parse(plan.weekStart))));