[Feature/Improvement] Migrate to audio_service from flutter_background_service

# Overview

This issue describes the plan to replace `flutter_background_service` with `audio_service` as the background execution mechanism.

The current implementation uses [flutter_background_service](https://pub.dev/packages/flutter_background_service). It uses [SQFlite](https://pub.dev/packages/sqflite) for passing interval data, [SharedPreferences](https://pub.dev/packages/shared_preferences) for the pause flag, and service events (`service.invoke("stop");`). Migrating to audio_service will replace these with a single Dart stream.

## audio_service

[Tutorial](https://github.com/ryanheise/audio_service/wiki/Tutorial).

`audio_service` is built for long-running audio. It registers the app with the OS as a media player, which grants it a foreground service slot and lock-screen controls without any extra configuration. The BaseAudioHandler subclass runs in the same Dart isolate as the UI, so the interval list can be passed by reference in memory.

## Design

`HiitAudioHandler` - Extends the `BaseAudioHandler` class. Defines pause/play/skip, playSound, broadcasts state. This will replace flutter_background_service.

`Countdown` - Stateful widget, contains the streambuilder.

`CountdownController` - Public-facing controller that translates user calls into handler methods.

`IntervalType` - Unchanged.

`TimerState` - Unchanged.