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_test.go

@@ -0,0 +1,65 @@
+// 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 (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOauth2ClientIDFromContinueURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		continueURL string
+		want        string
+	}{
+		{
+			name:        "extracts client_id from authorize URL",
+			continueURL: "/api/connect/v1/oauth2/authorize?client_id=https%3A%2F%2Ftrust.example.com%2F.well-known%2Foauth-client-metadata&response_type=code",
+			want:        "https://trust.example.com/.well-known/oauth-client-metadata",
+		},
+		{
+			name:        "returns empty for non-authorize URL",
+			continueURL: "/overview",
+			want:        "",
+		},
+		{
+			name:        "returns empty for path suffix lookalike",
+			continueURL: "/evil/oauth2/authorize?client_id=phishing",
+			want:        "",
+		},
+		{
+			name:        "returns empty when client_id is missing",
+			continueURL: "/api/connect/v1/oauth2/authorize?response_type=code",
+			want:        "",
+		},
+		{
+			name:        "extracts client_id from absolute authorize URL",
+			continueURL: "https://auth.example.com/api/connect/v1/oauth2/authorize?client_id=gid%3A%2F%2Fprobo%2Foauth2_client%2Fabc",
+			want:        "gid://probo/oauth2_client/abc",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			assert.Equal(t, tt.want, oauth2ClientIDFromContinueURL(tt.continueURL))
+		})
+	}
+}