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/components/file-viewer-page.js

@@ -65,12 +65,14 @@ export class FileViewerPage extends FileViewerBase {
             <h2 class="page-header-title fv-title" title=${this._path ?? ''}><bdi>${this._path ?? ''}</bdi></h2>
           </div>
           <div class="fv-header-actions">
+            ${this._renderHistoryButton('btn btn-sm btn-outline-secondary fv-download-btn')}
             ${this._renderModeToggle('btn btn-sm btn-outline-secondary fv-download-btn')}
             <button class="btn btn-sm btn-outline-secondary fv-download-btn" title=${t('fv.download')} @click=${() => this._download()}>
               <i class="bi bi-download"></i>
             </button>
           </div>
         </div>
+        ${this._renderVersionBanner()}
         <div class="fv-body">${this._renderBody()}</div>
       </div>
     `;