Follow-up to #188. Two additions to the side-panel file preview:
- Markdown files get a rendered/plain tabbed view (was: always
rendered, no way to see source) — same tab pattern as SVG.
- Raster images (png/jpg/gif/webp/bmp/ico/avif) render as an
<img>. /api/state-file previously from_utf8_lossy-stringified
every file and served text/plain, which corrupts binary; it
now serves image files as raw bytes with their real
content-type (over-cap images are rejected, not truncated —
a clipped binary is corrupt).
buildSvgPanel generalised to buildTabbedPreview, shared by SVG +
markdown. .svg-host/.svg-render renamed .preview-host/.img-preview
since they now back images + md too.
closes#192
f42ba9b561