fix: resolve all flutter analyze warnings and infos

- app.dart: add braces to single-statement if body
- briefing_api.dart: escape <id> in doc comment (unintended HTML)
- notes_api.dart, tasks_api.dart, projects_api.dart: use null-aware
  map elements (?'key': value) instead of if-null guards
- task_edit_screen.dart: remove unused tasks_api.dart import
- task_edit_screen.dart, project_selector.dart: suppress
  deprecated_member_use on DropdownButtonFormField.value (value is
  the controlled-widget param; switching to initialValue would break
  current-selection display)
- project_selector.dart: use _ instead of __ for ignored error params

lib/app.dart

@@ -371,7 +371,7 @@ class _QuickCaptureBarState extends ConsumerState<_QuickCaptureBar> {
 
   String _hintForLocation(String location) {
     if (location.startsWith(Routes.library) &&
-        location.contains('tasks')) return 'Add a task…';
+        location.contains('tasks')) { return 'Add a task…'; }
     if (location.startsWith(Routes.conversations)) return 'Ask Fabled…';
     return 'Capture a note…';
   }