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/uk.settings.ts

@@ -1459,6 +1459,8 @@ export const settingsDict = {
   "settings.mcp.page.status.label.failed": "Не вдалося",
   "settings.mcp.page.status.label.needsAuth": "Потрібна авторизація",
   "settings.mcp.page.status.label.needsRegistration": "Потрібна реєстрація",
+  "settings.mcp.page.status.label.awaitingRestart": "Очікує перезапуску",
+  "settings.mcp.page.status.description.awaitingRestart": "Сервер збережено, але ще не застосовано. Натисніть Apply & Restart, щоб OpenCode його завантажив — після цього стануть доступні підключення та авторизація.",
   "settings.mcp.page.status.description.connected": "Підключено, OpenCode може відкривати інструменти й ресурси.",
   "settings.mcp.page.status.description.failedDefault": "OpenCode не вдалося отримати доступ до цього сервера MCP.",
   "settings.mcp.page.status.description.needsAuth": "Цей віддалений сервер MCP потребує авторизації, перш ніж він зможе підключитися.",