🧑‍💻 Flutter flavors for multiple installed app instances

[Friedinger]

This pull request introduces support for multiple build flavors (development and production) in the Flutter app. The changes include updates to the build configuration, Gradle setup, and main application entry point to handle the new flavors. The most important changes are summarized below.

Build workflow updates:

• Updated .github/workflows/build.yml to include the --flavor option when building APKs and app bundles, and adjusted file paths to reflect the new production flavor. [1] [2] [3]

Gradle configuration changes:

• Added flavorDimensions and defined development and production product flavors in android/app/build.gradle. Each flavor has a unique applicationIdSuffix and app name.

Application-level updates:

• Modified AndroidManifest.xml to use a dynamic app name (@string/app_name) based on the selected flavor.
• Updated lib/main.dart to inject the current flavor (development or production) into the app using the Provider package.

Flutter configuration:

• Added a default-flavor configuration (development) in pubspec.yaml to set the default flavor for the app.

[Friedinger]

See https://github.com/RubberDuckCrew/gitdone/actions/runs/14810527854 for apk with production flavor.

Closes #56