feat(file-viewer): browse a versioned file's git history

A clock button in the file viewer header lists the versions of a file
whose project keeps a git history; picking one shows the file as of
that commit, read-only, with a banner back to the current version.

A past version is never served from the working tree: the whole
repository is materialized at that revision (git archive streamed
through tar into a size-bounded, immutable-by-rev cache) and every
fetch — content, compiled LaTeX, markdown images, downloads — resolves
inside that tree, so dependencies are contemporaneous with the file:
a .tex compiles against its \input's and images of that moment.

Backend: new git_versions module (repo discovery bounded by the
workspace mount, host-git log/rev-parse/archive, extraction cache with
oldest-first prune) + GET /api/file/versions and a rev param on
GET /api/file (rev is the ETag; never X-Writable). Frontend: history
mode in FileViewerBase shared by the desktop and mobile viewers —
popover, banner, watcher paused while browsing, rev propagated to
every /api/file URL it builds.

web/i18n/en.js

@@ -1040,6 +1040,10 @@ export default {
   'fv.zoom_in':             'Zoom in',
   'fv.zoom_out':            'Zoom out',
   'fv.pdf_failed':          'This PDF could not be displayed.',
+  'fv.history':             'History',
+  'fv.current':             'current',
+  'fv.version_banner':      'Version of {date} — read-only',
+  'fv.back_to_current':     'Back to current',
 
   // ── Marketplace ─────────────────────────────────────────────────────────────
   'marketplace.title':               'Marketplace',