fix(todo,app): Material's icon font where serverpod_auth_idp_flutter asks for it

serverpod_auth_idp_flutter sets `uses-material-design: true`, and Todo's
app and halkyon_app didn't: Flutter warned in every test run ("the
primary pubspec contains uses-material-design: false"), and Material's
icons would draw as boxes in Todo. Learn's app has it already. The lock
stays as it was.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/pubspec.yaml

@@ -79,6 +79,10 @@ dev_dependencies:
 flutter:
   # Generates HalkyonAppLocalizations from lib/l10n (see l10n.yaml).
   generate: true
+  # Material's icon font, which serverpod_auth_idp_flutter's widgets ask for
+  # (it sets this itself): without it every test run here warns, and its
+  # widgets' icons would draw as boxes in them (each app sets it for itself).
+  uses-material-design: true
 
   assets:
     # flag-icons' MIT notice, for the licence list (registerFlagIconsLicense).