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_en.arb

@@ -150,5 +150,15 @@
   "planSelectRange": "Select period",
   "planGenerateButton": "Plan",
   "planGenerating": "Generating plan\u2026",
-  "planSuccess": "Menu planned!"
+  "planSuccess": "Menu planned!",
+  "planProductsTitle": "Products for the menu",
+  "planProductsSubtitle": "AI will take the selected products into account when generating recipes",
+  "planProductsEmpty": "No products added",
+  "planProductsEmptyMessage": "Add products you have at home \u2014 AI will suggest recipes from what you already have",
+  "planProductsAddProducts": "Add products",
+  "planProductsContinue": "Continue",
+  "planProductsSkip": "Skip product selection",
+  "planProductsSkipNoProducts": "Plan without products",
+  "planProductsSelectAll": "Select all",
+  "planProductsDeselectAll": "Deselect all"
 }