Address PR review: custom typeRegistry test, CI Flutter version pin, example pubspec material design flag

[Copilot]

Three issues flagged in review on the publication-readiness PR:

• typeRegistry test was a no-op — the test claimed to verify EditorController.fromJson respects a custom typeRegistry but called it with the default registry. Replaced with a test that registers a _PrefixedHeaderMapper and asserts the custom-mapped text appears in the parsed block:

final customRegistry = BlockTypeRegistry()
  ..register(_PrefixedHeaderMapper('Custom: '));

final ctrl = EditorController.fromJson(json, typeRegistry: customRegistry);
expect((ctrl.blocks.first as HeaderBlock).text, 'Custom: Hi');

• CI Flutter version unpinned — subosito/flutter-action was set to channel: stable only, which can drift from the repo-pinned version. Added flutter-version: '3.41.5' to match .fvmrc.

• example/pubspec.yaml missing uses-material-design: true — the example app uses MaterialApp and Icons.* but the pubspec lacked the flutter: section required to bundle the Material Icons font. Added it.

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