[jokerttu]

Moves the plugin off package:flutter/material.dart, which Flutter deprecates after 3.47 in favor of package:material_ui (migration guide).

• lib/ imports package:flutter/widgets.dart instead. The only Material symbol it used was Colors.black as the default stroke and fill color of circles, polygons and polylines; those defaults are now const Color(0xFF000000), the same value.
• The plugin does not depend on material_ui, so apps can keep either flutter/material.dart or material_ui without type conflicts in the plugin's API. The public API and the minimum Flutter version (3.44) are unchanged.
• The plugin's own tests use material_ui as a dev dependency (^1.0.1, which resolves on Flutter 3.44), migrated with dart fix --apply --code=migrate_design_widgets.
• A new test pins the default colors as literals; it fails when a default changes.

Stacked on #793.

Checks run

• Flutter 3.44.9: flutter analyze --fatal-infos lib test clean, 268 Dart tests pass.
• Flutter 3.47.9: flutter analyze --fatal-infos lib test clean, 268 Dart tests pass.
• Formatting with both versions: no changes.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation
• I added new tests to check the change I am making
• All existing and new tests are passing.