Project task view: tapping a project shows milestone-grouped task list

- ProjectLibraryCard.onTap navigates to /projects/:id/tasks
- New ProjectTasksScreen: milestone sections with coloured dot,
  done/total counter and thin progress bar; unassigned tasks at bottom
- Status cycle icon updates optimistically (pendingStatus map) so
  the list doesn't flash on every tap; syncs global tasksProvider
- New route /projects/:id/tasks registered in app.dart
- TasksApi.getByProject + TasksRepository + projectTasksProvider (family)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/constants.dart

@@ -16,4 +16,5 @@ abstract class Routes {
   static const settings = '/settings';
   static const briefing = '/briefing';
   static const library = '/library';
+  static const projectTasks = '/projects/:id/tasks';
 }