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/.