Add adaptive theme based on ambient light sensor

[Michvista]

Closes #408
Automatically switch light/dark mode from Android lux readings with hysteresis and debounced consecutive readings, plus a Settings toggle and manual override when the user picks a theme.

Added an Adaptive Theme option that automatically switches between light and dark mode using the device ambient light sensor (Android via expo-sensors).

• New Adaptive Theme toggle under Settings → App
• Hysteresis thresholds: dark below 25 lux, light above 75 lux
• Debounced transitions: 3 consecutive readings, 1s apart, to avoid flicker
• Sensor subscription pauses when the app is backgrounded
• Manual override: picking a theme in the theme picker disables adaptive theme
• Unit tests for settings store and hook debounce/subscription lifecycle
• Documentation in docs/adaptive-theme.md

Test plan

• Run npm run test -- tests/store/settingsStore.test.ts tests/hooks/useAdaptiveTheme.test.ts
• On Android: enable Adaptive Theme, move between bright/dim light; confirm smooth theme changes without flicker
• Change Theme manually while adaptive is on; confirm adaptive toggle turns off
• Background and foreground the app; confirm no crashes and adaptive behavior resumes when enabled

Notes

LightSensor is Android-only. On iOS/web, the toggle is stored but has no sensor effect.

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