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/public/constants.js

@@ -1458,7 +1458,7 @@ function computeRewriteScrollLine(input) {
  * a `/g` regex, so {@link absoluteFilePathPattern} mints a fresh one per call.
  */
 const FILE_PATH_LINK_PATTERN =
-  /(\/(?:home|Users|tmp|var|private|opt|mnt|srv|media|data|workspace)\/[^\s"'<>|;&\n\x00-\x1f]*\.(?:log|txt|json|md|ya?ml|csv|xml|sh|py|tsx|ts|jsx|js|mjs|cjs|css|html|toml|ini|sql|png|jpe?g|gif|webp|bmp|svg|pdf|docx|pptx|mp4|webm|mov|mp3|wav))\b/g;
+  /(\/(?:home|Users|tmp|var|private|opt|mnt|srv|media|data|workspace)\/[^\s"'<>|;&\n\x00-\x1f]*\.(?:log|txt|json|md|ya?ml|csv|xml|sh|py|tsx|ts|jsx|js|mjs|cjs|css|html|toml|ini|sql|png|jpe?g|gif|webp|avif|bmp|ico|svg|pdf|docx|pptx|mp4|webm|mov|mp3|wav))\b/g;
 
 /** A fresh, zero-state instance of {@link FILE_PATH_LINK_PATTERN}. */
 function absoluteFilePathPattern() {
@@ -1476,7 +1476,7 @@ function absoluteFilePathPattern() {
  * file in /tmp played fine. test/media-extension-parity.test.ts pins the sync.
  */
 const FILE_PREVIEW_EXTENSIONS = new Set(
-  ('png jpg jpeg gif webp bmp svg pdf docx pptx mp4 webm mov m4v ogv mp3 wav ogg oga m4a aac flac opus').split(' ')
+  ('png jpg jpeg gif webp avif bmp ico svg pdf docx pptx mp4 webm mov m4v ogv mp3 wav ogg oga m4a aac flac opus').split(' ')
 );
 
 /** Whether a path's extension is one {@link FILE_PREVIEW_EXTENSIONS} covers. */