feat: add configurable default Pomodoro duration in Settings

[syyam]

Summary

Closes #70

Adds a Default Pomodoro Duration option under General settings. The user can pick from preset durations (5, 10, 15, 25, 30, 45, 60 minutes) via an animated chip row shown in a bottom sheet. The selected value is persisted in DataStore and survives app restarts.

Changes

File	What changed
SettingsStore.kt	New DEFAULT_POMODORO_DURATION_KEY DataStore key + getter/setter
MainState.kt	Added defaultPomodoroDuration: Int = 25
MainAction.kt	Added UpdateDefaultPomodoroDuration(mins: Int)
MainViewModel.kt	Collects key on init, handles action
PomodoroOptionComponent.kt	New bottom sheet component with animated chips
SettingsScreen.kt	Wires item + bottom sheet case
strings.xml	Three new string resources

Test plan

• Open Settings → General → "Default Pomodoro Duration"
• Bottom sheet opens with chip row showing 5/10/15/25/30/45/60 min
• Selecting a chip animates it (scale + color), dismisses sheet, and persists across restarts
• Default is 25 min if never configured
• All existing settings options still work