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_ja.dart

@@ -418,4 +418,35 @@ class AppLocalizationsJa extends AppLocalizations {
 
   @override
   String get planSuccess => 'メニューが計画されました！';
+
+  @override
+  String get planProductsTitle => 'メニューの食材';
+
+  @override
+  String get planProductsSubtitle => 'AIはレシピ生成時に選択した食材を考慮します';
+
+  @override
+  String get planProductsEmpty => '食材が追加されていません';
+
+  @override
+  String get planProductsEmptyMessage =>
+      '家にある食材を追加してください — AIが手持ちの食材でレシピを提案します';
+
+  @override
+  String get planProductsAddProducts => '食材を追加';
+
+  @override
+  String get planProductsContinue => '続ける';
+
+  @override
+  String get planProductsSkip => '食材選択をスキップ';
+
+  @override
+  String get planProductsSkipNoProducts => '食材なしでプランニング';
+
+  @override
+  String get planProductsSelectAll => 'すべて選択';
+
+  @override
+  String get planProductsDeselectAll => 'すべて解除';
 }