Extends the existing Engine-API-based docker integration adapter rather than
porting Termix's SSH+CLI approach, since ArchNest's docker integrations only
ever configure a baseUrl. Adds backend/src/docker/{client,exec}.ts and
backend/src/routes/docker.ts (REST + websocket exec-terminal via raw socket
hijack), and a new Containers page wired into the sidebar/router.
Verified end-to-end against a real dockerd instance and a real container in
this sandbox, which caught and fixed a genuine bug: calling /exec/{id}/resize
before starting the exec hangs the daemon indefinitely; fixed by setting the
initial size via ConsoleSize at exec-create time instead.
