Templates: Version the theme stylesheet so updates bypass the browser cache

[whereareiam]

After a theme update, pages rendered with the new templates but the previous theme stylesheet until the browser cache expired. #64 versioned the scripts and component stylesheets the templates link; this closes the gap it left for the main theme-*.css files.

Cause

Forgejo links the selected theme as theme-<name>.css?v=<Forgejo version> and serves it with Cache-Control: max-age=21600. A theme update does not change Forgejo's version, so the URL stays the same and browsers keep the cached file for up to six hours. The auto themes add a second layer: they @import their light and dark files by a fixed path.

Seen after deploying #70: the pull request list showed its new heading row and counters with the old layout rules (button beside the heading, icons on Open/Closed, old filter order).

Changes

• base/head_style.tmpl: the theme overrides Forgejo's two-line partial. It is identical except that the build appends a version of all theme files to the theme link: ?v={{AssetVersion}}-<theme version>. Forgejo's own index.css link is unchanged.
• Auto themes: each @import in theme-*-auto.css gets the content version of the file it loads. This is done in dist, so the release archives carry it too.
• Asset stamping: links that already carry a version are skipped, so the new partial's index.css link is left alone.

Limits

• Installations that use only the CSS archives and not the templates keep Forgejo's own link. The versioned imports still help them once the auto file itself is refetched.
• The first update that introduces this partial is still affected once, because the cached page links come from the previous build.
• The partial copies two lines of Forgejo 16.0.3's template; it has to be rechecked when Forgejo changes that file.

Checks

• bun commit checks pass (403 tests), including a new test that the theme link and every auto-theme import carry the version of the files they load. Running the assemble step twice gives the same output.
• Dev preview (Forgejo 16.0.3): pages link theme-github-auto.css?v=16.0.3~gitea-1.22.0-<version>, the stylesheet and its versioned imports load, no template errors are logged, and the pull request list renders as before.

ref #64

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