Admin Notification Dispatcher: Template Registry

Define a `NotificationTemplateType` enum for the 4 admin template types; use it as the registry key for O(1) lookup. Model each template as an immutable Dart class with `const` constructor: `NotificationTemplate({ required this.titleTemplate, required this.bodyTemplate, required this.notificationType, required this.targetRole, required this.requiredVariables })`. Implement variable substitution as a pure function: `String render(String template, Map variables)` using `String.replaceAll()` in a loop over `requiredVariables` — do not use a full regex engine for this. Validate variables at registry construction time by checking that every `{{variable}}` pattern in the template strings has a corresponding entry in `requiredVariables`.

For Norwegian pluralization, implement a minimal `pluralize(int count, String singular, String plural)` helper rather than pulling in an i18n library for two forms. Wrap the shared `NotificationService` with an `AdminNotificationService` facade so admin-specific concerns (template resolution, org-scoped recipients) are separated from the low-level FCM dispatch.

Unit tests: instantiate the template registry with all 4 templates and assert no TemplateValidationException is thrown. Test each template's variable substitution with valid inputs and assert the output string matches expected format. Test plural form handling for PENDING_REIMBURSEMENT_SUMMARY with count=1 and count=3. Test that a template with an undefined variable throws TemplateValidationException at registration.

Test recipient resolution: mock OrgHierarchyService and the user query; assert only users with the correct role and within the subtree are returned. Localization test: assert Norwegian strings are returned when org locale is 'nb', English when 'en'. Integration test: register templates, call recipient resolution against a Supabase local instance with seeded users, assert the returned recipient list matches expected user IDs.