Adds a workspace background image that renders behind all UI, configured
via a new `background` settings section (path, fit, opacity).
The image is loaded asynchronously with a 2-second timeout and is never
read on the render path, so an unreachable source (e.g. a dead network
mount) cannot block or freeze the workspace. The first successful read is
cached under the data directory (written once, never rewritten), and when
the source is unavailable the cached image is served from that cache, so
the background keeps working offline.
yenru0
4c94072003