Route connect OAuth through unified sign-in

Drop the portal-login redirect, accept continue URLs for magic links,
brand emails from the OAuth client when present, and update e2e coverage.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/server/api/connect/v1/oauth2_client_id.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package connect_v1
+
+import (
+	"net/url"
+)
+
+const oauth2AuthorizeFullPath = "/api/connect/v1" + oauth2AuthorizePath
+
+func oauth2ClientIDFromContinueURL(continueURL string) string {
+	parsed, err := url.Parse(continueURL)
+	if err != nil {
+		return ""
+	}
+
+	// Only read client_id from the real authorize endpoint. The continue URL
+	// itself is already validated by saferedirect before this runs.
+	if parsed.Path != oauth2AuthorizeFullPath {
+		return ""
+	}
+
+	return parsed.Query().Get("client_id")
+}