fix: palette changes now propagate immediately via ThemeContext

Replace useSetting()-based useTheme() with a React context (ThemeProvider
+ useTheme/usePaletteControl). The context holds palette key in state so
pressing a palette button in Settings re-renders all screens instantly.
Persists to SQLite and reloads the stored value on mount.

mobile/theme.ts

@@ -1,5 +1,3 @@
-import { useSetting } from '@/db/queries';
-
 export type PaletteKey = 'auto' | 'default' | 'giro' | 'tour' | 'vuelta';
 
 export const PALETTES = {
@@ -29,8 +27,3 @@ export function autoKey(): Exclude<PaletteKey, 'auto'> {
   return 'default';
 }
 
-export function useTheme(): Theme {
-  const override = (useSetting('palette_override') ?? 'auto') as PaletteKey;
-  const key = override === 'auto' ? autoKey() : override;
-  return PALETTES[key as keyof typeof PALETTES] ?? PALETTES.default;
-}