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/css/file-viewer.css

@@ -260,6 +260,102 @@
   flex-shrink: 0;
 }
 
+/* ── History mode (git versions) ───────────────────────────────────────────── */
+
+.fv-history {
+  position: relative;
+  display: inline-flex;
+}
+
+/* Transparent click-catcher that closes the popover on outside click. */
+.fv-history-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 1040;
+}
+
+.fv-history-pop {
+  position: absolute;
+  top: calc(100% + 6px);
+  right: 0;
+  z-index: 1050;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  min-width: 260px;
+  max-width: min(340px, 80vw);
+  max-height: 320px;
+  overflow-y: auto;
+  padding: 4px;
+  background: var(--card-bg);
+  border: 1px solid var(--card-border);
+  border-radius: var(--radius-md);
+  box-shadow: var(--card-shadow);
+}
+
+.fv-history-row {
+  display: flex;
+  flex-direction: column;
+  gap: 1px;
+  padding: 0.45rem 0.6rem;
+  text-align: left;
+  background: none;
+  border: 0;
+  border-radius: var(--radius-sm);
+  color: var(--bs-body-color);
+  cursor: pointer;
+}
+.fv-history-row:hover { background: var(--sidebar-hover); }
+.fv-history-row.active { background: var(--sidebar-active-bg); }
+
+.fv-history-date {
+  display: flex;
+  align-items: center;
+  gap: 0.4rem;
+  font-size: 0.72rem;
+  color: var(--bs-secondary-color);
+}
+
+.fv-history-current {
+  padding: 0 0.35rem;
+  font-size: 0.66rem;
+  border-radius: 999px;
+  background: var(--accent-soft);
+  color: var(--accent);
+}
+
+.fv-history-subject {
+  font-size: 0.8rem;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+/* Banner shown between header and body while viewing a past revision. */
+.fv-version-banner {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.6rem;
+  padding: 0.45rem 0.85rem;
+  background: var(--bs-info-bg-subtle, #cff4fc);
+  border-bottom: 1px solid var(--bs-info-border-subtle, #9eeaf9);
+  color: var(--bs-emphasis-color, inherit);
+  flex-shrink: 0;
+}
+.fv-version-banner > .bi {
+  color: var(--bs-info-text-emphasis, #055160);
+  flex-shrink: 0;
+}
+.fv-version-text {
+  flex: 1 1 200px;
+  min-width: 0;
+  font-size: 0.82rem;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
 /* ── PDF preview ─────────────────────────────────────────────────────────────── */
 
 /* PDFs are drawn by <pdf-view> (web/components/shared/pdf-view.js) on canvas, not