feat: persist app-wide appearance

[mkaltner]

Summary

• persist Light, Dark, and Auto appearance through the app-owned preferences
• apply theme changes immediately at the ArcaneTheme root, including system-bar icon contrast
• keep theme writes alive across Appearance-screen navigation while preserving accent settings
• fall back safely to Auto for missing or invalid stored values

Validation

• ./gradlew :app:testDebugUnitTest :app:assembleDebug
• 153 unit tests passed; debug APK assembled
• 5 focused appearance mapping, persistence, and ownership tests passed
• physical-device checks passed immediate Light/Dark switching, Back/re-entry and rapid-departure persistence, force-stop restoration, accent retention, Auto system-mode tracking, and system-bar readability