chore: adopt FVM, migrate to Melos 7, upgrade dependencies

[juanRodriguez17]

Description

Brings the boilerplate up to date. Three connected changes:

• FVM: pin Flutter SDK to 3.41.3 via .fvmrc. CI reads the version from the same file (subosito/flutter-action). VS Code is wired to .fvm/flutter_sdk. README documents the new contributor flow.
• Melos 7 / Dart pub workspaces: workspace is declared in the root pubspec.yaml, every package sets resolution: workspace, the old melos.yaml files are removed. Use melos bootstrap going forward — there is now a single shared pubspec.lock at the repo root.
• Dependencies: major bumps for bloc/flutter_bloc (8 → 9), bloc_test (9 → 10), get_it (7 → 8), permission_handler (11 → 12), flutter_lints (3 → 5), melos (3 → 7). Patch bumps across dio, intl_utils, firebase_core, equatable, universal_html, shared_preferences, build_runner, mocktail. Dropped unused deps: url_strategy, dev, http, mockito, and melos as a project dep.

Cleanup

• Removed deprecated dart_code_metrics block from analysis_options.yaml.
• Deleted orphan modules/notifications/ (no pubspec.yaml, not referenced anywhere).
• Removed the flutter create counter-template test stub.
• Lint fixes forced by the analyzer: super-parameters, withValues replaces withOpacity, context.mounted guard, doc-comment escapes.
• Sonarqube workflow uncommented and aligned to the FVM-pinned Flutter version.

Setup for reviewers

After pulling the branch:

```
dart pub global activate fvm
fvm install
dart pub global activate melos
melos bootstrap
```

Verification

• melos run analyze
• melos run format
• melos exec --dir-exists=test -- flutter test

Notes

• No new feature work or architectural changes.