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/de.settings.ts

@@ -1417,6 +1417,8 @@ export const settingsDict = {
   'settings.mcp.page.status.label.failed': 'Fehlgeschlagen',
   'settings.mcp.page.status.label.needsAuth': 'Autorisierung erforderlich',
   'settings.mcp.page.status.label.needsRegistration': 'Registrierung erforderlich',
+  'settings.mcp.page.status.label.awaitingRestart': 'Wartet auf Neustart',
+  'settings.mcp.page.status.description.awaitingRestart': 'Dieser Server ist gespeichert, aber noch nicht übernommen. Verwenden Sie „Apply & Restart“, damit OpenCode ihn lädt — danach sind Verbindung und Autorisierung verfügbar.',
   'settings.mcp.page.status.description.connected': 'Verbunden und bereit, Tools und Ressourcen zu entdecken.',
   'settings.mcp.page.status.description.failedDefault': 'OpenCode konnte diesen MCP-Server nicht erreichen.',
   'settings.mcp.page.status.description.needsAuth': 'Dieser entfernte MCP-Server erfordert Autorisierung, bevor eine Verbindung hergestellt werden kann.',