fix: render agent-embedded images for non-default agents on web and desktop

Agent replies embed workspace-relative image refs (e.g. images/x.png) that
only resolved for the default agent. Backend now rewrites relative media refs
to absolute /api/file URLs on the SSE/polling and history paths, and the
desktop markdown renderer recognizes those URLs and prefixes them with the
backend base. Also add a native right-click Save/Copy image menu on desktop.

Co-authored-by: cowagent <cow@cowagent.ai>