[fix] Find/create dark mode logo/favicon

[LauraLangdon]

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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