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.

test/routes/file-routes.test.ts

@@ -681,6 +681,18 @@ describe('file-routes', () => {
       expect(body.data.url).toContain('file-raw');
     });
 
+    it('classifies avif as an image so the viewer renders it instead of dumping bytes', async () => {
+      mockedStat.mockResolvedValue({ size: 1024 } as never);
+
+      const res = await harness.app.inject({
+        method: 'GET',
+        url: `/api/sessions/${harness.ctx._sessionId}/file-content?path=photo.avif`,
+      });
+      expect(res.statusCode).toBe(200);
+      const body = JSON.parse(res.body);
+      expect(body.data.type).toBe('image');
+    });
+
     it('returns audio metadata for audio files', async () => {
       mockedStat.mockResolvedValue({ size: 2048 } as never);
 
@@ -817,6 +829,19 @@ describe('file-routes', () => {
       expect(res.headers['content-type']).toBe('image/png');
     });
 
+    it('serves avif with its image type, since <img> refuses an octet-stream', async () => {
+      const content = Buffer.from('fake avif data');
+      mockedCreateReadStream.mockReturnValue(Readable.from([content]) as never);
+      mockedStat.mockResolvedValue({ size: content.length } as never);
+
+      const res = await harness.app.inject({
+        method: 'GET',
+        url: `/api/sessions/${harness.ctx._sessionId}/file-raw?path=photo.avif`,
+      });
+      expect(res.statusCode).toBe(200);
+      expect(res.headers['content-type']).toBe('image/avif');
+    });
+
     it('serves workspace SVG as an untrusted attachment instead of inline image/svg+xml', async () => {
       const content = Buffer.from('<svg><script>alert("xss")</script></svg>');
       mockedCreateReadStream.mockReturnValue(Readable.from([content]) as never);