fix: file viewer opens audio/svg/binary like the attachments viewer

The File Browser preview and Attachments preview share openFilePreview(),
but the workspace branch (via /file-content) misclassified several types the
attachments viewer handled fine:

- SVG was reported as type:image, but file-raw serves SVG as octet-stream +
  attachment (XSS hardening), so the <img> broke. Now fetched and rendered via
  a same-origin image/svg+xml blob <img> (safe; <img> never runs SVG scripts).
  file-raw's SVG hardening is unchanged.
- Audio (mp3/wav/ogg/m4a/aac/flac/opus) was type:binary -> "Cannot preview".
  Now classified as audio and rendered with <audio controls>; file-raw gained
  the matching audio/video MIME types so playback works.
- Binary formats not in the hardcoded list (xlsx/doc/zip/...) were decoded as
  UTF-8 and dumped as mojibake. Replaced the static list with a NUL-byte
  content sniff that flags arbitrary binaries; the binary fallback now offers a
  Download link instead of dead-ending.

Adds route tests for audio, known-binary (xlsx), and NUL-sniff classification.
Verified end-to-end on an isolated instance + headless browser.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/styles.css

@@ -7239,12 +7239,22 @@ kbd {
   max-height: 100%;
 }
 
+.file-preview-body audio {
+  display: block;
+  width: 90%;
+  margin: 2rem auto;
+}
+
 .file-preview-body .binary-message {
   padding: 2rem;
   text-align: center;
   color: var(--text-muted);
 }
 
+.file-preview-body .binary-message a {
+  color: var(--accent, #4ea1ff);
+}
+
 .file-preview-footer {
   padding: 0.35rem 0.75rem;
   font-size: 0.7rem;