Upgrade unhead to v3

[holtwick]

Describe the bug

unhead is required in frontend code as well and therefore a dependency in the package.json's dependencies section. Vite-SSG still is on v2.

https://unhead.unjs.io/releases

Reproduction

This leads to issues, i.e. when using htmlAttrs: {lang:

System Info

System:
    OS: macOS 26.4.1
    CPU: (10) arm64 Apple M4
    Memory: 112.27 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.14.1 - /opt/homebrew/opt/node@24/bin/node
    npm: 11.11.0 - /opt/homebrew/opt/node@24/bin/npm
    pnpm: 10.33.0 - /opt/homebrew/bin/pnpm
    bun: 1.3.12 - /Users/dirk/.bun/bin/bun
    Deno: 2.3.5 - /Users/dirk/.deno/bin/deno
  Browsers:
    Brave Browser: 146.1.88.138
    Firefox: 149.0.2
    Safari: 26.4

Used Package Manager

pnpm

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A, please open a GitHub Discussion instead.
• The provided reproduction is a minimal reproducible of the bug.

Contributions

• I am willing to submit a PR to fix this issue
• I am willing to submit a PR with failing tests (actually just go ahead and do it, thanks!)