[fix] Find/create dark mode logo/favicon

The current DA logo (navy blue on transparent) doesn't work well on dark backgrounds, so I've disabled dark mode in #80. The main site also only has a light-mode logo, so this may need to be created from scratch or requested from the design team.

**Current state:**
- Theme toggle is disabled (ThemeSelect component overridden with an empty component)
- Site is forced to light mode via CSS `color-scheme: light`

**To re-enable dark mode:**
1. Add a dark-mode logo SVG to `src/assets/`
2. Update `astro.config.mjs` to use Starlight's `logo.dark` option
3. Remove the `ThemeSelect` override in `astro.config.mjs`
4. Remove the `color-scheme: light` line from `src/styles/custom.css`
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Start with astro.config.mjs, src/styles/custom.css, and the existing files in src/assets/. Check how the ThemeSelect override, color-scheme declaration, and Starlight logo configuration currently work. Done means a dark-mode logo SVG is available, the dark logo is configured, the override and forced light mode are removed, and dark mode works again.

Written by the indexing model from the issue text.