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_de.dart

@@ -422,4 +422,36 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get planSuccess => 'Menü geplant!';
+
+  @override
+  String get planProductsTitle => 'Zutaten für den Speiseplan';
+
+  @override
+  String get planProductsSubtitle =>
+      'Die KI berücksichtigt die ausgewählten Produkte bei der Rezeptgenerierung';
+
+  @override
+  String get planProductsEmpty => 'Keine Produkte hinzugefügt';
+
+  @override
+  String get planProductsEmptyMessage =>
+      'Füge Produkte hinzu, die du zu Hause hast — die KI schlägt Rezepte aus deinen Vorräten vor';
+
+  @override
+  String get planProductsAddProducts => 'Produkte hinzufügen';
+
+  @override
+  String get planProductsContinue => 'Weiter';
+
+  @override
+  String get planProductsSkip => 'Produktauswahl überspringen';
+
+  @override
+  String get planProductsSkipNoProducts => 'Ohne Produkte planen';
+
+  @override
+  String get planProductsSelectAll => 'Alle auswählen';
+
+  @override
+  String get planProductsDeselectAll => 'Alle abwählen';
 }