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_it.dart

@@ -422,4 +422,36 @@ class AppLocalizationsIt extends AppLocalizations {
 
   @override
   String get planSuccess => 'Menu pianificato!';
+
+  @override
+  String get planProductsTitle => 'Prodotti per il menu';
+
+  @override
+  String get planProductsSubtitle =>
+      'L\'AI terrà conto dei prodotti selezionati nella generazione delle ricette';
+
+  @override
+  String get planProductsEmpty => 'Nessun prodotto aggiunto';
+
+  @override
+  String get planProductsEmptyMessage =>
+      'Aggiungi prodotti che hai in casa — l\'AI suggerirà ricette con quello che hai già';
+
+  @override
+  String get planProductsAddProducts => 'Aggiungi prodotti';
+
+  @override
+  String get planProductsContinue => 'Continua';
+
+  @override
+  String get planProductsSkip => 'Salta la selezione dei prodotti';
+
+  @override
+  String get planProductsSkipNoProducts => 'Pianifica senza prodotti';
+
+  @override
+  String get planProductsSelectAll => 'Seleziona tutto';
+
+  @override
+  String get planProductsDeselectAll => 'Deseleziona tutto';
 }