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/menu/handler.go

@@ -31,6 +31,8 @@ type UserLoader interface {
 // ProductLister returns human-readable product lines for the AI prompt.
 type ProductLister interface {
 	ListForPrompt(ctx context.Context, userID string) ([]string, error)
+	// ListForPromptByIDs returns only the products with the given IDs.
+	ListForPromptByIDs(ctx context.Context, userID string, ids []string) ([]string, error)
 }
 
 // RecipeSaver creates a dish+recipe and returns the new recipe ID.
@@ -121,9 +123,10 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 	}
 
 	var body struct {
-		Week      string   `json:"week"`       // optional, defaults to current week
-		Dates     []string `json:"dates"`      // YYYY-MM-DD; triggers partial generation
-		MealTypes []string `json:"meal_types"` // overrides user preference when set
+		Week       string   `json:"week"`        // optional, defaults to current week
+		Dates      []string `json:"dates"`       // YYYY-MM-DD; triggers partial generation
+		MealTypes  []string `json:"meal_types"`  // overrides user preference when set
+		ProductIDs []string `json:"product_ids"` // when set, only these products are passed to AI
 	}
 	_ = json.NewDecoder(r.Body).Decode(&body)
 
@@ -136,7 +139,7 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if len(body.Dates) > 0 {
-		h.generateForDates(w, r, userID, u, body.Dates, body.MealTypes)
+		h.generateForDates(w, r, userID, u, body.Dates, body.MealTypes, body.ProductIDs)
 		return
 	}
 
@@ -150,8 +153,14 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 
 	menuReq := buildMenuRequest(u, locale.FromContext(r.Context()))
 
-	if products, productError := h.productLister.ListForPrompt(r.Context(), userID); productError == nil {
-		menuReq.AvailableProducts = products
+	if len(body.ProductIDs) > 0 {
+		if products, productError := h.productLister.ListForPromptByIDs(r.Context(), userID, body.ProductIDs); productError == nil {
+			menuReq.AvailableProducts = products
+		}
+	} else {
+		if products, productError := h.productLister.ListForPrompt(r.Context(), userID); productError == nil {
+			menuReq.AvailableProducts = products
+		}
 	}
 
 	days, generateError := h.menuGenerator.GenerateMenu(r.Context(), menuReq)
@@ -194,7 +203,7 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 // generateForDates handles partial menu generation for specific dates and meal types.
 // It groups dates by ISO week, generates only the requested slots, upserts them
 // without touching other existing slots, and returns {"plans":[...]}.
-func (h *Handler) generateForDates(w http.ResponseWriter, r *http.Request, userID string, u *user.User, dates, requestedMealTypes []string) {
+func (h *Handler) generateForDates(w http.ResponseWriter, r *http.Request, userID string, u *user.User, dates, requestedMealTypes, productIDs []string) {
 	mealTypes := requestedMealTypes
 	if len(mealTypes) == 0 {
 		// Fall back to user's preferred meal types.
@@ -212,8 +221,14 @@ func (h *Handler) generateForDates(w http.ResponseWriter, r *http.Request, userI
 
 	menuReq := buildMenuRequest(u, locale.FromContext(r.Context()))
 	menuReq.MealTypes = mealTypes
-	if products, productError := h.productLister.ListForPrompt(r.Context(), userID); productError == nil {
-		menuReq.AvailableProducts = products
+	if len(productIDs) > 0 {
+		if products, productError := h.productLister.ListForPromptByIDs(r.Context(), userID, productIDs); productError == nil {
+			menuReq.AvailableProducts = products
+		}
+	} else {
+		if products, productError := h.productLister.ListForPrompt(r.Context(), userID); productError == nil {
+			menuReq.AvailableProducts = products
+		}
 	}
 
 	weekGroups := groupDatesByWeek(dates)

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) {