Replace the bespoke index.css with a shadcn-vue-idiomatic theme.css
(Catppuccin Latte/Mocha as the default), and add a palette picker to the
profile sheet that swaps between 6 alternative palettes scoped under
:root[data-theme="<name>"]: Countryside Castle, Dark Matter, Emerald
Forest, Light Green, Neo Brutalist, Starry Night.
useTheme() now also persists a 'ui-palette' localStorage key alongside
the existing 'ui-theme' (dark/light/system) and applies the choice via a
data-theme attribute on <html>. Standalone apps inherit the palette
automatically since AppShell already invokes useTheme() on mount.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
