build: move the pinned toolchain to Flutter 3.47.5

[luisburgos]

Moves the pinned toolchain from Flutter 3.44.0 to 3.47.5 (Dart 3.13.4): .fvmrc, which local fvm and the CI jobs read, the license check's separately pinned flutter_version, and the pre-push hook's comment naming the version.

The pubspec's flutter: ">=3.41.0" lower bound is unchanged. It constrains the package's consumers, not this repository's toolchain, so no release is needed.

Flutter 3.47's tooling added build/** and web/** to the analyzer excludes on first run. Those edits are kept, since they only stop the analyzer from reading generated output.

Two lints are new under the 3.47.5 analyzer and each fails flutter analyze, so they are fixed here:

• unnecessary_unawaited in FlowinChipGroupViewPager: 3.47 annotates PageController.animateToPage with @awaitNotRequired, so the unawaited wrapper is dropped. The call is unchanged at runtime.

• matching_super_parameters in the showcase's ThemeModeController: ValueNotifier's positional parameter is now _value, so the super parameter takes that name. Writing : super(initial) instead trips use_super_parameters.

• lefthook run pre-push --all-files passes on 3.47.5: format, analyze, tests, spell-check and the version-bump guard.

• Every sub-package passes pub get, format and analyze, plus tests where it has them.

• Line coverage from flutter test --coverage stays at 100%.

• The published showcase still builds with --wasm.
  Not verified: the very_good test --min-coverage 100 gate itself, since very_good_cli is not installed locally. That is this pull request's own CI.

The very_good_workflows references stay pinned at v1.21.0, which also works on Dart 3.13. Dependabot's bump to v1.21.1 becomes safe to accept once this merges.