Make ClerkProvider appearance reactive to OS theme changes

[coderabbitai]

Context

This issue tracks a follow-up improvement suggested during PR #187 review.

Currently, the ClerkProvider in src/App.tsx uses a static browser.prefersDarkMode() call to determine the appearance theme. This means the theme won't update if the user changes their OS theme while the app is running.

Task

Replace the static browser.prefersDarkMode() call with MUI's useMediaQuery hook to make the theme reactive to OS theme changes.

References

• PR: fix clerk provider usage #187
• Comment: fix clerk provider usage #187 (comment)
• Requested by: @narthur