UI docs index still calls the theme model "currently dark-only"

## What

The UI documentation index describes the theme model as "currently dark-only". The app has shipped a runtime light theme with a toggle for some time, so the line sends a reader looking for light-theme guidance away from it.

## Where

`docs/ui/README.md` line 7:

```
| [Theming](theming.md) | Tailwind v4 `@theme inline`, shadcn/ui CSS variables, the color palette, and the (currently dark-only) theme model |
```

`src/components/theme-toggle.tsx` and `src/components/theme-provider.tsx` both exist, and `docs/ui/theming.md` itself describes the app as dark-first with a runtime light theme. The index row is the stale one.

## Expected

The row describes the theme model as it is: dark-first with a runtime light theme.

## How to fix

1. Open `docs/ui/README.md`.
2. On line 7, replace `the (currently dark-only) theme model` with `the dark-first theme model with a runtime light theme`.
3. While you are in there, read `docs/ui/theming.md` and confirm the index row still matches what that page says. If any other row in the table has drifted, fix it in the same pull request.

## How to check

Read `docs/ui/README.md` and `docs/ui/theming.md` side by side. The index row should no longer contradict the page it points at.

Open docs/ui/README.md and compare its theme index row with docs/ui/theming.md. Check the referenced theme-toggle.tsx and theme-provider.tsx files if needed to confirm the runtime light theme. Done means the row says the theme is dark-first with a runtime light theme and no longer contradicts the linked page.

Written by the indexing model from the issue text.