feat: play a subtle tick at each minute boundary during Pomodoro

[syyam]

Summary

Closes #87

Plays a short, low-volume audio cue at every minute mark while the Pomodoro timer is counting down. This gives the user an ambient signal that time is passing without requiring them to look at the screen.

Implementation

• Uses Android's built-in ToneGenerator (TONE_CDMA_ABBR_ALERT, 120 ms, 40% volume on STREAM_NOTIFICATION) — no raw audio asset needed.
• Added SoundEvent.POMODORO_TICK to the SoundEvent enum and a playMinuteTick() private helper in AudioUtil.kt.
• PomodoroService injects SettingsStore in onCreate() to track the soundEnabled preference, then fires the tick when newLeft % 60 == 0L.
• Gated entirely by the existing Enable In-App Sounds setting — no new UI controls.

Test plan

• Start a Pomodoro session; at each minute boundary a soft double-beep plays
• Disabling "Enable In-App Sounds" silences the tick
• No tick fires on pause or after completion
• Existing task-added/completed/deleted sounds still work