Add battery status screen

[ghostyapps]

Summary

• implement a Jetpack Compose UI that shows battery percentage, last full charge time, discharge duration, and an estimated remaining time card
• add a view model that listens to battery change broadcasts and persists the last full charge timestamp for calculations
• introduce a simple Compose theme and supporting color palette along with coroutine support for state flows

Testing

• ./gradlew :app:assembleDebug (fails: Android SDK is not available in the execution environment)

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Codex Task