fix(ui): pause host-proxy site when its dev-server worker is stopped (502 fix)

[prabowosd]

Fixes #671.

Problem

Stopping a host-proxy site's dev server from the dashboard leaves the site returning 502 Bad Gateway for every request. Reproduced on a secured host-proxy site running a dev server (e.g. pnpm dev): after stopping its app worker the dev-server port is no longer listening, yet a request to the site returns 502.

The vhost is still the live proxy, pointing at the now-dead port:

location / {
    proxy_pass http://host.containers.internal:<dev-port>;
    ...
}

Root cause

A host-proxy site's dev server is the synthesized app worker (config.HostProxyWorkerName), not a framework worker. The dashboard's worker:app:stop action calls WorkerStopForSite, which tears down only the lerd-app-<site> unit. Nothing swaps the proxy vhost, so nginx keeps proxying to the dead port and 502s.

It is also asymmetric: because app is not in any framework's Workers map, the worker:app:start path bails with "worker app not defined", so once stopped there is no per-worker way to bring it back, only lerd start or pause/unpause.

PauseSite already does the right thing for host-proxy sites (GeneratePausedVhost swaps the secured vhost to the paused page and removes the stale -ssl.conf, then reloads), and UnpauseSite restores the proxy vhost and restarts the dev server. Stopping the only thing a host-proxy site runs is, semantically, pausing the site.

Fix

Route the parent app worker's start/stop through PauseSite / UnpauseSite at the dashboard handler (so there is no recursion through WorkerStopForSite):

• worker:app:stop (parent, host-proxy) -> PauseSite: swaps the vhost to the paused landing page, stops the dev server, marks the site paused. The paused page's Resume button then works because the registry state is consistent.
• worker:app:start (parent, host-proxy) -> UnpauseSite: restores the proxy vhost and dev server.
• Other workers, worktree targets (branch set), and non-host-proxy sites keep the plain per-worker path untouched.

The decision is factored into a pure hostProxyAppLifecycleOp helper so the routing is unit-testable without standing up the HTTP handler.

Worktree host-proxy dev servers (worker:app:stop?branch=...) have the same class of issue but no single-worktree pause primitive exists; left for a follow-up and explicitly excluded here (they keep the normal worker path).

Test

TestHostProxyAppLifecycleOp (table-driven) pins the routing: parent app stop -> pause, start -> unpause, and untouched for non-host-proxy sites, other workers, worktree targets, and unknown ops. go build + go vet ./internal/ui clean.