dashboard file preview: markdown tabs + raster image rendering

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

hive-c0re/assets/dashboard.css

@@ -327,16 +327,16 @@ code {
   border-color: var(--purple);
   background: rgba(203, 166, 247, 0.08);
 }
-/* SVG file preview (issue #188) */
-.svg-host { margin-top: 0.5em; }
-.svg-render {
+/* Image / tabbed file preview (issues #188, #192) */
+.preview-host { margin-top: 0.5em; }
+.img-preview {
   display: block;
   max-width: 100%;
   height: auto;
   margin: 0 auto;
   border: 1px solid var(--border);
   border-radius: 4px;
-  /* checkerboard so transparent regions of the SVG read clearly */
+  /* checkerboard so transparent regions of the image read clearly */
   background: repeating-conic-gradient(#313244 0% 25%, #1e1e2e 0% 50%) 50% / 18px 18px;
 }
 .approval-tabs {