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.

web: hide the per-row diff scrollbar gutter (overlay style)

Each @pierre/diffs code row scrolls horizontally on its own
(overflow-x: scroll) and reserves a classic scrollbar track sized to the
measured --diffs-scrollbar-gutter. On platforms with non-overlay
scrollbars (e.g. macOS "always show"), that track shows as a grey strip
under any row with a long line, reading as leftover bottom padding.

Set --diffs-scrollbar-gutter-override: 0 on #diff-root so the track is
never reserved; long lines stay scrollable by trackpad / wheel. Verified
the override wins over a simulated 14px measured gutter (reserved height
stays 0).

Re-hashed the embedded stylesheet.

web: remove grey padding bands inside each diff file

Two sources put a grey strip above the first line and below the last,
inside the file border:

- The shared `base` partial's `section { padding-bottom: .5rem }` applied
  to .diff-file (it is a <section>). Cancel it with padding-bottom: 0.
- The @pierre/diffs vertical code padding (--diffs-gap-block, default 8px)
  above the first and below the last line. Set it to 0 on #diff-root; it
  inherits into the DIFFS-CONTAINER shadow DOM. Horizontal code padding
  (--diffs-gap-inline) is left on its default.

Re-hashed the embedded stylesheet.

web: tighten diff and file-tree spacing

Address three spacing complaints in the compare view:

- File tree: switch @pierre/trees to the "compact" density preset (24px
  rows, 0.8 factor) so folder/child rows read as a dense index rather
  than an airy list.
- Diff blocks: drop the inter-file gap (.diff-file margin-bottom
  1rem -> .5rem) and the tree/diff column gap (1rem -> .75rem).
- File tree height: size the mount to its content height (capped at the
  viewport) instead of always filling the viewport, removing the band of
  empty tree background below the last file. Re-measures on resize and on
  every tree change (expand/collapse) via tree.subscribe.

Rebuilt and re-hashed the embedded bundle and stylesheet.

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/.