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/en.settings.ts

@@ -1482,6 +1482,8 @@ export const settingsDict = {
   'settings.mcp.page.status.label.failed': 'Failed',
   'settings.mcp.page.status.label.needsAuth': 'Needs auth',
   'settings.mcp.page.status.label.needsRegistration': 'Needs registration',
+  'settings.mcp.page.status.label.awaitingRestart': 'Awaiting restart',
+  'settings.mcp.page.status.description.awaitingRestart': 'This server is saved but not applied yet. Use Apply & Restart to load it into OpenCode — connect and authorization become available after that.',
   'settings.mcp.page.status.description.connected': 'Connected and ready for OpenCode to discover tools and resources.',
   'settings.mcp.page.status.description.failedDefault': 'OpenCode could not reach this MCP server.',
   'settings.mcp.page.status.description.needsAuth': 'This remote MCP server requires authorization before it can connect.',