Fix docs search visual quality

[niemyjski]

Summary

• refine the docs sidebar search trigger and replace the oversized search strip with a centered command-palette layout
• add a visible heading and close control while preserving keyboard navigation and focus restoration
• version the custom stylesheet with the deployment version so new search markup cannot be paired with stale CSS
• keep the search experience full-screen and usable on mobile

Verification

• EXCEPTIONLESS_SITE_VERSION=visual-quality-test deno task verify
• desktop browser dogfood at 1280x720 with real search results
• mobile browser dogfood at 390x844
• verified Escape focus restoration, close-button body unlock, and clean browser console

Breaking changes

None.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Exceptionless.AppHost	38%	39%	144	❌
Exceptionless.Core	72%	64%	8854	➖
Exceptionless.Insulation	23%	23%	205	❌
Exceptionless.Web	84%	67%	6841	✔
Summary	76% (21372 / 28148)	65% (9794 / 15158)	16044	✔