feat(reporter): support html reporter single file output

[hi-ogawa]

Description

• Closes Option to save report in a single combined HTML file #6425
• Plan https://gist.github.com/hi-ogawa/a12115ae69358e9a68ad7f9ede0923fd

I was hoping the same service worker trick as https://github.com/vitest-dev/viewer might work, but it requires at least two index.html and sw.js. So manual assets/data inlining logic needs to live inside html reporter itself.

It should be doable but we need a few independent new logic, namely:

• inlining SPA
• inlining html.meta.json.gz
• inlining attachments from path to body
• inlining MPA (for coverage iframe) (this will be a separate follow-up)
  • actually coverage spa reporter should make this simpler

Interestingly, the logic required for SPA and MPA inlining are specfic to Vitest html report, so I might look for existing tools or just vibe-code from scratch.

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