fix(providers): complete OAuth logins that finish in the browser

OpenCode's authorize response reports how the client must finish: `code`
expects a pasted code, while `auto` requires the client to call
oauth/callback immediately and hold it open — upstream blocks in there
polling for the device code or waiting on its loopback redirect, and only
that call persists the credential. Every auth plugin OpenCode ships uses
`auto`; none use `code`.

The page implemented only `code`. It opened the browser, showed a paste
field no provider can fill, and never called back, so a successful sign-in
stored nothing and the app sat unchanged. Authorization now drives the UI:
`auto` chains straight into the callback behind a waiting state with a
cancel, and the paste field appears only when a provider actually asks
for a code.

Two smaller failures shared that surface. Prompts were never collected,
which put GitHub Copilot Enterprise out of reach entirely, so a method
that declares them now asks first and passes the answers to authorize.
Device codes are also recovered from the instructions text, where they
actually live — the old code read fields the API does not return, so the
copy button never appeared.

The callback is exempt from the ordinary proxy deadline and gets a
15-minute budget, bounded by the shortest upstream expiry we know of.
A human sign-in with 2FA does not fit in four minutes, and expiring it
turned a completed login into a 504.

packages/web/server/lib/opencode/DOCUMENTATION.md

@@ -375,6 +375,8 @@ an authoritative loopback callback URL even when OpenChamber binds port `0`.
   - SSE forwarders: `GET /api/global/event`, `GET /api/event`
     - Downstream heartbeats keep clients and intermediaries alive, while a separate upstream-only stall watchdog closes the downstream response when OpenCode stops producing bytes so clients reconnect instead of trusting synthetic heartbeats indefinitely. Each watchdog reset uses the current load-aware timeout, matching the shared event transport.
   - Session message forwarder: `POST /api/session/:sessionId/message`
+  - Interactive OAuth forwarder: `POST /api/provider/:providerID/oauth/callback`
+    - Upstream blocks inside this call for the whole browser sign-in (device-code polling or a loopback redirect), so it is exempt from the ordinary request deadline and uses a 15-minute proxy timeout instead of `LONG_REQUEST_TIMEOUT_MS`. All other `/api/provider/*` routes, including `oauth/authorize`, keep the ordinary deadline.
   - Generic `/api/*` forwarding with hop-by-hop header filtering
   - Windows `/session` merge fallback path behavior
   - OpenCode readiness gate for proxied `/api` requests