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_zh.dart

@@ -418,4 +418,34 @@ class AppLocalizationsZh extends AppLocalizations {
 
   @override
   String get planSuccess => '菜单已规划！';
+
+  @override
+  String get planProductsTitle => '菜单食材';
+
+  @override
+  String get planProductsSubtitle => 'AI在生成食谱时会考虑所选食材';
+
+  @override
+  String get planProductsEmpty => '尚未添加食材';
+
+  @override
+  String get planProductsEmptyMessage => '添加您家中的食材 — AI将根据您已有的食材推荐食谱';
+
+  @override
+  String get planProductsAddProducts => '添加食材';
+
+  @override
+  String get planProductsContinue => '继续';
+
+  @override
+  String get planProductsSkip => '跳过食材选择';
+
+  @override
+  String get planProductsSkipNoProducts => '不选食材直接规划';
+
+  @override
+  String get planProductsSelectAll => '全选';
+
+  @override
+  String get planProductsDeselectAll => '取消全选';
 }