feat: add dark mode variant for app icon

[jvsena42]

This PR adds dark mode support for the app icon, displaying a black background when the device is in dark mode instead of the default orange.

Description

• Adds a new launcher_bg color resource that resolves to orange (#FF4400) in light mode and black (#000000) in dark mode
• Updates all launcher icons (mainnet, testnet, regtest) to use the new launcher_bg color instead of hardcoded brand color
• Updates splash screen theme to also respect dark mode with matching background

Preview

dark.webm

QA Notes

1. Install the app on a device running Android 8+
2. In light mode, verify the app icon shows an orange background
3. Switch device to dark mode (Settings > Display > Dark theme)
4. Verify the app icon now shows a black background
5. Test on mainnet, testnet, and regtest flavors if possible

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[jvsena42]

Android launchers don't reliably support night-qualified resources for icon backgrounds

[jvsena42]

Drafted because the icon wasn't changing when switching between dark/light modes. Tested on emulator, will test on physical device