fix: respond to media query change events when theme is auto

[techfg]

Description

• Closes auto theme does not respond to change in system/browser preferences #1684
• What does this PR change? Give us a brief description.
  Will respond to media change query events and update the theme to system/browser preference when current theme is set to auto
• Did you change something visual? A before/after screenshot can be helpful.
  Technically it's a visual change but nothing visual about the package changed :)

I didn't see any existing tests in/around the components, specifically ThemeSelect. If I overlooked and/or you'd like me to add some, just let me know.

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🦋 Changeset detected

Latest commit: 19d163f

The changes in this PR will be included in the next version bump.

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@astrojs/starlight	Patch

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[delucis]

Thank you for this PR @techfg! I made a couple of small tweaks — mainly to use the existing #loadTheme() method instead of writing additional logic for extracting the theme from the select menu.

There is a small “bug” in this version of things because we actually have two instances of <ThemeSelect> in most pages — one in the nav bar, one in the mobile menu. That means we have two media query event listeners that, due to some internal wiring, each update both instances. I’ll tackle that separately though as it will need a small refactor to pull some of this logic up out of the custom element class.

Appreciate you helping out 🙌

[techfg]

@delucis - Thanks for reviewing so quickly and getting merged! Your changes make sense, much simpler, and yeah, noticed the multiple ThemeSelect but figured that was a separate issue as you mention :)