ci: add automated Flutter app health verification

[alamin-karno]

Motivation

• The repository was configured for FVM but there was no verification that the app builds or tests successfully after that tooling change.
• Add an automated, repeatable check so PRs and pushes validate app health and catch regressions early.

Description

• Add a GitHub Actions workflow .github/workflows/flutter-ci.yml that sets up Flutter (stable) and runs flutter pub get, flutter analyze, flutter test, and flutter build apk --debug.
• Update README.md to document local verification steps using FVM with the commands fvm flutter --version, fvm flutter pub get, fvm flutter analyze, fvm flutter test, and fvm flutter run.
• Commit the workflow and README changes on a verification branch so CI will start running these checks on push and PRs.

Testing

• Attempted to download a Flutter SDK binary with curl but the environment blocked outbound downloads (HTTP 403 CONNECT tunnel failed).
• Attempted to validate YAML with a small Python check (yaml.safe_load) but the container lacks PyYAML, so the validation did not run.
• No local flutter/fvm commands were executed because this environment does not have Flutter/FVM installed; the added CI workflow will run flutter pub get, flutter analyze, flutter test, and flutter build apk --debug on GitHub Actions for automated verification.

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Codex Task