Re-enables the previously scaffolded-but-disabled theme system (fixes loadSelectedTheme hard-coding light, wires up React Navigation's DarkTheme), defaults new installs to dark, adds a quick theme picker to the Hub screen matching the Currency picker UX, and applies dark styling to Hub cards/icons, the bottom tab bar, and the shared header used by Hub/Account/Change Password/Withdraw screens.
Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com