Correct the Dart SDK constraint in pubspec.yaml

**Problem**

`pubspec.yaml` declares an SDK floor that the lockfile cannot satisfy. A contributor who installs the declared minimum cannot resolve dependencies, and CI has no correct version to pin to.

**Where**

- `pubspec.yaml:17` declares `sdk: ">=3.1.0 <4.0.0"`
- `pubspec.lock` `sdks:` records `dart: ">=3.9.0 <4.0.0"` and `flutter: ">=3.35.0"`

**Expected**

The declared constraint and the lockfile agree.

**Fix sketch**

- Set `sdk: ">=3.9.0 <4.0.0"` and note the Flutter floor in the README.

**Verification**

The `flutter pub get` step of CI proves the two agree. Confirm it fails if the floor is set back to 3.1.0 while the lock still demands 3.9.0.

Open pubspec.yaml at line 17 and compare its Dart SDK floor with the sdks entries in pubspec.lock. Update the declared constraint and document the Flutter floor in the README, then run the flutter pub get CI step. Done means dependency resolution succeeds and the constraints agree, including the stated failure condition when the floor is reverted.

Written by the indexing model from the issue text.