React Navigation paints every scene with its own theme background and defaults
to the light one, so a screen that did not paint `bg` itself sat on white while
the text tokens stayed light. Root-level background + a matching navigator theme
means no screen can fall through to a light surface again, and the status bar
icons are legible on the dark palette.
Every screen but Login set `backgroundColor: bg` itself, so the login page and
the boot loader fell through to the platform's white surface while the text
tokens stayed light — white on light, unreadable. The palette in
`theme/tokens.ts` is fixed dark by construction, and `Uniwind.setTheme` defaulted
to light, so heroUI's own surfaces were light too. Paint the background at the
root, on the login screen and on the loader, and default the theme to dark.
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