fix(mcp-oauth): allowlist Cursor's OAuth callbacks so its dynamic registration succeeds (#2225)

* fix(mcp-oauth): allowlist Cursor's OAuth callbacks so its dynamic registration succeeds

Cursor (IDE, CLI, and Grok Bot on top of it) registers three redirect URIs
in one /register request: cursor://anysphere.cursor-mcp/oauth/callback,
https://www.cursor.com/agents/mcp/oauth/callback and
http://localhost:8787/callback. Only the loopback matched a built-in
pattern and /register fails the whole set on any unknown URI, so every
Cursor connection to the URL we hand out in Settings died with
"Redirect URI not allowed". Users cannot self-register the cursor://
form either (the settings panel requires https).

Add a built-in `cursor` provider with the two non-loopback callbacks as
exact matches (no cursor.com prefix), name it "Cursor (Anysphere)" on
the consent page, list the pre-approved clients in the OAuth clients
settings text (sv + en) and the mcp-server rules, and cover the
register, allowlist and consent paths with tests.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DBFeTvQXgMCNXR6fG9drff

* fix(mcp-oauth): show the cursor:// deeplink unverified and let CSP pass its post-consent redirect

Review findings on #2225, one pass:

- Skeptic (correctness), REFUTED: new URL('cursor://...').origin is the
  string "null", so the consent page emitted form-action 'self' null and
  Chromium would block the 303 to the deeplink after Allow. The header
  now uses the scheme-source (cursor:) when the origin is opaque; a test
  pins the header on the cursor:// URI.
- Skeptic (security), CodeRabbit (Major) and Superagent (P2): a custom
  scheme can be claimed by any local app (RFC 8252 section 8.4), so it
  must not be presented as a vendor-verified callback. The deeplink is
  its own provider, cursor_deeplink, rendered "Cursor (Anysphere)" with
  the localhost tag "Din egen dator" and verified: false. The https
  cursor.com callback keeps the verified label. A test pins that a code
  minted without a code_challenge can never be exchanged, which is what
  keeps a scheme hijack from turning into a token.
- CodeRabbit (Minor): the rules doc now says the Grok callback matches
  with or without the trailing slash.
- Regression skeptic: docs/WHITELABEL.md listed only Claude and
  localhost and pointed at the wrong file; now lists the built-ins and
  points at lib/auth/oauth-allowlist.ts.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DBFeTvQXgMCNXR6fG9drff

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

docs/WHITELABEL.md

@@ -100,7 +100,7 @@ A few things that look brand-related but are configured elsewhere:
 - **Supabase auth emails** (password reset, magic link): set in the Supabase dashboard for your project, not in code.
 - **Resend sending domain**: verify `noreply@your-brand.se` (or wherever) in Resend, set `RESEND_FROM_EMAIL`.
 - **DNS / domain**: point `app.your-brand.se` at your Vercel deployment.
-- **OAuth redirect allowlist for MCP**: `app/api/mcp-oauth/authorize/route.ts` lists `claude.ai/api/*`, `claude.com/api/*`, and localhost. Your domain is the OAuth issuer, not a redirect target: no change needed unless you're integrating with new MCP clients.
+- **OAuth redirect allowlist for MCP**: `lib/auth/oauth-allowlist.ts` has built-in entries for Claude (`claude.ai/api/*`, `claude.com/api/*`), ChatGPT, Grok, Cursor and localhost; anything else is registered per user under Settings > API & MCP > OAuth clients. Your domain is the OAuth issuer, not a redirect target: no change needed unless you're integrating with new MCP clients.
 - **iCal feed PRODID** (`lib/calendar/ics-generator.ts`): defaults to `erp-base.se`, callers may pass their domain.
 - **`NEXT_PUBLIC_APP_URL`**: used as the OAuth issuer and safe auth-link fallback. For a dedicated one-brand deployment, set this to your domain (e.g. `https://app.your-brand.se`). For a shared hosted deployment, keep the canonical main app URL here and register additional hosts through `NEXT_PUBLIC_WHITELABEL_DOMAINS`.
 - **Skatteverket submission identity**: `extensions/general/skatteverket/lib/api-client.ts` does not set a custom `User-Agent`; submissions go out with the Node/Vercel runtime default. If your deployment needs to identify itself to Skatteverket under a different brand, that's a future enhancement (env var + header), not something the current branding service covers.