SVG files in the side-panel file preview showed only raw source.
Add a rendered/source tabbed view: 'rendered' (default) shows the
image, 'source' shows the markup.
The image loads via an <img> data: URI — <img>-loaded SVG runs in
the browser's secure static mode (scripts + external fetches
disabled), so an untrusted SVG from an agent's state dir can't
execute code in the dashboard origin. Tabs reuse the existing
diff-base-tab styling; a checkerboard backs the image so
transparent regions read clearly.
closes#188
a8ab91ecd8