feat: add timer presets and completion state

[qxbyte]

Summary

• add 1/5/10/25 minute presets to the timer screen
• show live running, paused, and completed status copy
• present a completion alert as soon as the countdown reaches zero

Testing

• xcodebuild -project Alarm.xcodeproj -scheme Alarm -destination 'generic/platform=iOS' -derivedDataPath /private/tmp/alarm-derived build (blocked by signing profile for org.xue.Alarm)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep completion state separate from alert visibility

Using didComplete as the isPresented binding for .alert causes SwiftUI to reset it to false as soon as the user dismisses the alert, so the timer immediately leaves the completed state and falls through to the paused copy (remainingSeconds == 0 but didComplete == false). This makes the new completion status non-persistent and misleading right after acknowledgement; use a separate showCompletionAlert flag so completion status can remain true until reset/restart.

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