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.dart

@@ -927,6 +927,66 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Menu planned!'**
   String get planSuccess;
+
+  /// No description provided for @planProductsTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Products for the menu'**
+  String get planProductsTitle;
+
+  /// No description provided for @planProductsSubtitle.
+  ///
+  /// In en, this message translates to:
+  /// **'AI will take the selected products into account when generating recipes'**
+  String get planProductsSubtitle;
+
+  /// No description provided for @planProductsEmpty.
+  ///
+  /// In en, this message translates to:
+  /// **'No products added'**
+  String get planProductsEmpty;
+
+  /// No description provided for @planProductsEmptyMessage.
+  ///
+  /// In en, this message translates to:
+  /// **'Add products you have at home — AI will suggest recipes from what you already have'**
+  String get planProductsEmptyMessage;
+
+  /// No description provided for @planProductsAddProducts.
+  ///
+  /// In en, this message translates to:
+  /// **'Add products'**
+  String get planProductsAddProducts;
+
+  /// No description provided for @planProductsContinue.
+  ///
+  /// In en, this message translates to:
+  /// **'Continue'**
+  String get planProductsContinue;
+
+  /// No description provided for @planProductsSkip.
+  ///
+  /// In en, this message translates to:
+  /// **'Skip product selection'**
+  String get planProductsSkip;
+
+  /// No description provided for @planProductsSkipNoProducts.
+  ///
+  /// In en, this message translates to:
+  /// **'Plan without products'**
+  String get planProductsSkipNoProducts;
+
+  /// No description provided for @planProductsSelectAll.
+  ///
+  /// In en, this message translates to:
+  /// **'Select all'**
+  String get planProductsSelectAll;
+
+  /// No description provided for @planProductsDeselectAll.
+  ///
+  /// In en, this message translates to:
+  /// **'Deselect all'**
+  String get planProductsDeselectAll;
 }
 
 class _AppLocalizationsDelegate