rename the service to diff.sr.ht

The instance-facing name changes and nothing else does: the config.ini section,
the origin, the apk package, the nginx block and the systemd unit are all
diff.sr.ht now, and the brand's red service label reads "diff" because chrome
derives it from the section name.

What deliberately stays is everything whose rename would cost more than it is
worth: the Go module path, the repository name, the comparesrht binary, and the
/~owner/repo/compare/base...head routes people have in their history. README
says so at the top so the split is not read as an oversight.

frontend: pierre diffs/trees bundle and sourcehut theme css

Vendored esbuild bundle (frontend/src/app.ts) rendering @pierre/diffs
FileDiff per file and a @pierre/trees FileTree sidebar against the SSR
#compare-data contract, plus scss/main.scss (base + compare views)
compiled and content-hashed into web/static/.