[fix](client): paint the app background and default the theme to dark

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.

src/screens/LoginScreen.tsx

@@ -12,7 +12,7 @@ import {
   Screen,
   TextInputBox,
 } from '../components/ui';
-import { space4, text, textMuted, textSecondary, typeCaption, typeLabel, typeTitle } from '../theme/tokens';
+import { space4, text, textMuted, textSecondary, typeCaption, typeLabel, typeTitle, bg } from '../theme/tokens';
 
 type Mode = 'signin' | 'register';
 
@@ -54,7 +54,7 @@ export default function LoginScreen() {
   };
 
   return (
-    <View style={{ flex: 1 }}>
+    <View style={{ flex: 1, backgroundColor: bg }}>
       {!online ? <OfflineStrip /> : null}
       <KeyboardAvoidingView
         style={{ flex: 1 }}