fix(dart): Widen preact_signals dependency constraint in a2ui_core to support ^7.0.0

### Description
Currently, `dart/a2ui_core/pubspec.yaml` pins `preact_signals` to:
```yaml
dependencies:
preact_signals: ^1.9.4
```

When integrating `a2ui_core` with modern signal-based architectures and Flutter packages that depend on `preact_signals: ^7.0.0` (such as `signals_core` 7.x or `bloc_signals`), Dart pub version solving fails:
```text
Because bloc_signals >=0.2.4 depends on preact_signals ^7.0.0 and a2ui_core >=0.0.1-dev002 depends on preact_signals ^1.9.4, bloc_signals >=0.2.4 is incompatible with a2ui_core >=0.0.1-dev002.
```

### Proposed Solution
Widen the dependency constraint in `dart/a2ui_core/pubspec.yaml` to:
```yaml
dependencies:
preact_signals: ">=1.9.4 <8.0.0"
# or: ^1.9.4 || ^7.0.0
```

The reactive signal primitives used by `a2ui_core` (`signal`, `computed`, `batch`) maintain full API compatibility across 1.9.x and 7.x.

Would the maintainers be open to a pull request with this change?