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/l10n/app_localizations_en.dart

@@ -420,4 +420,36 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get planSuccess => 'Menu planned!';
+
+  @override
+  String get planProductsTitle => 'Products for the menu';
+
+  @override
+  String get planProductsSubtitle =>
+      'AI will take the selected products into account when generating recipes';
+
+  @override
+  String get planProductsEmpty => 'No products added';
+
+  @override
+  String get planProductsEmptyMessage =>
+      'Add products you have at home — AI will suggest recipes from what you already have';
+
+  @override
+  String get planProductsAddProducts => 'Add products';
+
+  @override
+  String get planProductsContinue => 'Continue';
+
+  @override
+  String get planProductsSkip => 'Skip product selection';
+
+  @override
+  String get planProductsSkipNoProducts => 'Plan without products';
+
+  @override
+  String get planProductsSelectAll => 'Select all';
+
+  @override
+  String get planProductsDeselectAll => 'Deselect all';
 }