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>

backend/internal/domain/userproduct/repository.go

@@ -161,6 +161,46 @@ func (r *Repository) ListForPrompt(requestContext context.Context, userID string
 	return lines, rows.Err()
 }
 
+func (r *Repository) ListForPromptByIDs(requestContext context.Context, userID string, ids []string) ([]string, error) {
+	rows, queryError := r.pool.Query(requestContext, `
+		WITH up AS (
+			SELECT name, quantity, unit,
+			       (added_at + storage_days * INTERVAL '1 day') AS expires_at
+			FROM user_products
+			WHERE user_id = $1 AND id = ANY($2)
+		)
+		SELECT name, quantity, unit, expires_at
+		FROM up
+		ORDER BY expires_at ASC`, userID, ids)
+	if queryError != nil {
+		return nil, fmt.Errorf("list user products by ids for prompt: %w", queryError)
+	}
+	defer rows.Close()
+
+	var lines []string
+	now := time.Now()
+	for rows.Next() {
+		var name, unit string
+		var qty float64
+		var expiresAt time.Time
+		if scanError := rows.Scan(&name, &qty, &unit, &expiresAt); scanError != nil {
+			return nil, fmt.Errorf("scan user product for prompt: %w", scanError)
+		}
+		daysLeft := int(expiresAt.Sub(now).Hours() / 24)
+		line := fmt.Sprintf("- %s %.0f %s", name, qty, unit)
+		switch {
+		case daysLeft <= 0:
+			line += " (expires today ⚠)"
+		case daysLeft == 1:
+			line += " (expires tomorrow ⚠)"
+		case daysLeft <= 3:
+			line += fmt.Sprintf(" (expires in %d days ⚠)", daysLeft)
+		}
+		lines = append(lines, line)
+	}
+	return lines, rows.Err()
+}
+
 // --- helpers ---
 
 func scanUserProduct(row pgx.Row) (*UserProduct, error) {