Add seasonal availability dates to product form modal

- Add available_from and available_until fields to ProductFormValues type
- Add date picker inputs for seasonal availability in ProductFormModal
- Wire availability dates through ProductsClient to modal initial values
- Create migration 149 for database columns (already applied)

supabase/migrations/149_product_availability_dates.sql

@@ -0,0 +1,9 @@
+-- Add seasonal availability date fields to products table
+-- Allows products to have start/end availability dates (e.g., "Available mid-July through mid-September")
+
+ALTER TABLE products ADD COLUMN IF NOT EXISTS available_from TIMESTAMPTZ;
+ALTER TABLE products ADD COLUMN IF NOT EXISTS available_until TIMESTAMPTZ;
+
+-- Add comment for documentation
+COMMENT ON COLUMN products.available_from IS 'Start date for product availability (seasonal)';
+COMMENT ON COLUMN products.available_until IS 'End date for product availability (seasonal)';