web: full-width split diff view with a rendered file tree

Make the commit and compare pages full-bleed and default to the side-by-side
(split) diff, so the layout matches a conventional diff viewer: file tree on
the left, split diff on the right.

- layout.html renders the content wrapper with a per-page ContainerClass;
  chrome() defaults it to "container" and the commit/compare handlers set
  "container-fluid" for the diff views (the nav stays centered).
- app.ts defaults DEFAULT_LAYOUT to "split".
- Fix the @pierre/trees sidebar rendering blank: its virtualized container
  collapses to zero height unless the mount has a definite height, so
  sizeTreeRoot() gives #tree-root a viewport-height panel (re-applied on
  resize). Rebuilt the vendored bundle.

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/.