feat: prefer discounted transportation services

addresses #869axbjbz

docs/PROJECT_OVERVIEW.md

@@ -43,10 +43,18 @@
 - **Field Handlers**: 15+ specialized handlers in `src/Form/Service/`
   - Registered via service tags with dependency resolution
   - Process in dependency order via `ParticipantFieldHandlerRegistry`
+  - `ParticipantDateOfBirthFieldHandler` handles array input from BirthdayType widget
 - **Conditional Fields**: Universal condition system (`FieldConditionInterface`)
   - Age-based, field-dependent, service-specific conditions
   - Applied via `CreateFieldStateProvider` / `EditFieldStateProvider`
 - **HTMX Integration**: Real-time updates for dynamic fields
+- **Date of Birth Field**: Uses `BirthdayType` with three text inputs (day, month, year)
+  - Widget: `widget: 'text'` renders separate text fields instead of dropdowns
+  - Input format: `input: 'datetime_immutable'` for immutable date handling
+  - Custom theme: `birthday_widget` block in `templates/forms.html.twig` handles rendering
+  - German format: Day.Month.Year with hardcoded placeholders ("Tag", "Monat", "Jahr")
+  - Field order explicitly set to `['day', 'month', 'year']` for German date convention
+  - HTMX attributes applied to parent container for form refresh on change
 
 ### Service Layer (`src/Service/`)
 - `BookingService` - Core booking workflow
@@ -213,6 +221,8 @@ Critical for correct pricing and auto-reassignment:
 - Courses, additional services, board, insurance hidden until DOB provided
 - Age evaluated at travel start date, not current date
 - Dual constraint types: absolute_age, birth_year, mixed
+- **Array input handling**: `ParticipantDateOfBirthFieldHandler` processes array `['day' => X, 'month' => Y, 'year' => Z]` from three-field widget
+- **Date normalization**: Handler converts array to `DateTimeImmutable` using `sprintf()` with proper formatting
 
 ### Bulk Insurance Booking
 - Applicant enables bulk → applies to all participants