Fix demos dark mode

[zahlekhan]

Summary

• keep the demos code panel dark in dark mode instead of flipping to a white inverted surface
• strengthen muted text, input borders, panel borders, and empty-state contrast
• add an accessible label to the model selector

Screenshot

Root cause

The code panel used --openui-inverted-background and accent text tokens. Those tokens intentionally invert with the active theme, so the code canvas became white in dark mode while its text and controls remained styled for a dark canvas. Several tertiary text and border tokens also compounded low alpha values, making placeholders, empty states, and panel boundaries difficult to see.

Impact

The demos playground now has consistent dark surfaces and a clearer, more readable visual hierarchy. Light mode also benefits from stronger muted-text contrast.

Validation

• pnpm --filter @openuidev/docs types:check
• targeted ESLint checks
• Prettier and git diff --check