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/ui/src/lib/i18n/messages/es.settings.ts

@@ -1459,6 +1459,8 @@ export const settingsDict = {
   "settings.mcp.page.status.label.failed": "Fallido",
   "settings.mcp.page.status.label.needsAuth": "Necesita autenticación",
   "settings.mcp.page.status.label.needsRegistration": "Necesita registro",
+  "settings.mcp.page.status.label.awaitingRestart": "Esperando reinicio",
+  "settings.mcp.page.status.description.awaitingRestart": "Este servidor está guardado pero aún no aplicado. Usa «Apply & Restart» para cargarlo en OpenCode; después estarán disponibles la conexión y la autorización.",
   "settings.mcp.page.status.description.connected": "Conectado y listo para que OpenCode descubra herramientas y recursos.",
   "settings.mcp.page.status.description.failedDefault": "OpenCode no pudo alcanzar este servidor MCP.",
   "settings.mcp.page.status.description.needsAuth": "Este servidor MCP remoto requiere autorización antes de poder conectarse.",