fix(mcp): reliable OAuth across runtimes and honest pre-restart UI

MCP authorization was broken in several stacked ways. The browser return
leg landed on the SPA behind the auth gate, so the system browser saw a
login page instead of finishing; the pending-context store silently
saved nothing because its route had no JSON body parser; and the
callback-URL config write started deferring behind Apply & Restart, so
authorization ran against a runtime without the URL and dead-ended on
OpenCode's loopback listener.

The return leg is now completed entirely server-side by an
unauthenticated GET /mcp/oauth/callback that only forwards a code whose
state matches a parked context. Desktop with the local server and VS
Code switch to OpenCode's native flow over its fixed loopback port —
no config writes or restarts at all, with a one-time cleanup of the
previously written callback URL — and its completion signal drives the
page instead of blind status polling. Remote, hosted-web, and mobile
keep the server-callback flow, applying a queued callback-URL write
immediately since authorization cannot wait for a manual restart.

Also: a server queued behind Apply & Restart now shows an Awaiting
restart badge and explanation instead of connect/reauthorize buttons
that can only fail, and Reauthorize is offered only while the server is
actually connected.

packages/web/server/lib/opencode/proxy.js

@@ -839,5 +839,9 @@ export const registerOpenCodeProxy = (app, deps) => {
 
   app.use('/api', applyProxyResponseDeadline);
   app.post('/api/provider/:providerID/oauth/callback', interactiveOAuthProxy);
+  // OpenCode's native MCP OAuth flow: the request blocks until the user
+  // finishes authorization in the browser (up to OpenCode's 5-minute callback
+  // timeout), so it needs the interactive-OAuth deadline, not the default one.
+  app.post('/api/mcp/:name/auth/authenticate', interactiveOAuthProxy);
   app.use('/api', apiProxy);
 };