Moves src/backend/ai and its routes into plugins/ai, mounted through a
small /ai dispatcher instead of its own port. Moves the ai-enabled and
ai-private-endpoints settings routes out of user-settings-routes.ts
into the plugin too, so it has no leftover core dependents.
Moves docker backend and frontend into plugins/docker/, alongside
ssh-terminal. Adds a host editor tab seam, a dashboard card registry,
and a build step to compile plugin backends into dist.
LukeGus