feat(files): render markdown in the File Viewer, with Lines/Wrap toggles

Clicking a .md in the Files panel showed wrapped source with an Edit
pencil and no way to see it rendered, although marked + DOMPurify were
already on the page for the Response Viewer. The viewer now renders
.md/.markdown through that same pipeline (one parser, one click
delegate) with an MD pill back to source, and the plain-text view gains
Lines (CSS-counter gutter) and Wrap toggles. All three persist per device
in their own localStorage keys.

- Relative images are rebased onto the workspace-confined file-raw route
  under the document's directory, built inside a <template> so no fetch
  fires before the rewrite; a failed load degrades to alt text. Relative
  links become a.rv-path so the existing delegate opens them in the
  viewer; fragment and http(s) links are untouched.
- The rendered container carries data-i18n-skip so the translator does
  not rewrite the document's prose.
- Markdown fetches the route's 10000-line ceiling; other text keeps 500.
- avif renders inline (file-content image set, file-raw MIME map), and
  avif/ico printed paths open the viewer instead of tailing bytes. .md
  deliberately stays with the tail viewer for printed paths.

src/web/routes/file-routes.ts

@@ -1724,7 +1724,7 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
       // breadth of formats the attachments viewer renders (image/audio/video/pdf)
       // so the file viewer can open the same files.
       const ext = filePath.split('.').pop()?.toLowerCase() || '';
-      const imageExts = new Set(['png', 'jpg', 'jpeg', 'gif', 'webp', 'svg', 'bmp', 'ico']);
+      const imageExts = new Set(['png', 'jpg', 'jpeg', 'gif', 'webp', 'avif', 'svg', 'bmp', 'ico']);
       // Shared with the attachment registry so a video plays the same whether it
       // sits in the workspace or is reached by id from outside it.
       const videoExts = VIDEO_ATTACHMENT_EXTENSIONS;
@@ -2045,6 +2045,7 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
         jpeg: 'image/jpeg',
         gif: 'image/gif',
         webp: 'image/webp',
+        avif: 'image/avif',
         ico: 'image/x-icon',
         bmp: 'image/bmp',
         mp4: 'video/mp4',