fix(coderd): render HTML error page for OIDC email validation failures (#23059)

## Summary

When the email address returned from an OIDC provider doesn't match the
configured allowed domain list (or isn't verified), users previously saw
raw JSON dumped directly in the browser — an ugly and confusing
experience during a browser-redirect flow.

This PR replaces those JSON responses with the same styled static HTML
error page already used for group allow-list errors, signups-disabled,
and wrong-login-type errors.

## Changes

### `coderd/userauth.go`
Replaced 3 `httpapi.Write` calls in `userOIDC` with
`site.RenderStaticErrorPage`:

| Error case | Title shown |
|---|---|
| Email domain not in allowed list | "Unauthorized email" |
| Malformed email (no `@`) with domain restrictions | "Unauthorized
email" |
| `email_verified` is `false` | "Email not verified" |

All render HTTP 403 with `HideStatus: true` and a "Back to login" action
button.

### `coderd/userauth_test.go`
- Updated `AssertResponse` callbacks on existing table-driven tests
(`EmailNotVerified`, `NotInRequiredEmailDomain`,
`EmailDomainForbiddenWithLeadingAt`) to verify HTML Content-Type and
page content.
- Extended `TestOIDCDomainErrorMessage` to additionally assert HTML
rendering.
- Added new `TestOIDCErrorPageRendering` with 3 subtests covering all
error scenarios, verifying: HTML doctype, expected title/description,
"Back to login" link, and absence of JSON markers.

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

coderd/userauth.go

@@ -44,6 +44,7 @@ import (
 	"github.com/coder/coder/v2/coderd/util/ptr"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/cryptorand"
+	"github.com/coder/coder/v2/site"
 )
 
 type MergedClaimsSource string
@@ -1343,12 +1344,21 @@ func (api *API) userOIDC(rw http.ResponseWriter, r *http.Request) {
 		verified, ok := verifiedRaw.(bool)
 		if ok && !verified {
 			if !api.OIDCConfig.IgnoreEmailVerified {
-				httpapi.Write(ctx, rw, http.StatusForbidden, codersdk.Response{
-					Message: fmt.Sprintf("Verify the %q email address on your OIDC provider to authenticate!", email),
+				site.RenderStaticErrorPage(rw, r, site.ErrorPageData{
+					Status:     http.StatusForbidden,
+					HideStatus: true,
+					Title:      "Email not verified",
+					Description: fmt.Sprintf(
+						"Verify the %q email address on your OIDC provider to authenticate!",
+						email,
+					),
+					Actions: []site.Action{
+						{URL: "/login", Text: "Back to login"},
+					},
 				})
 				return
 			}
-			logger.Warn(ctx, "allowing unverified oidc email %q")
+			logger.Warn(ctx, "allowing unverified oidc email", slog.F("email", email))
 		}
 	}
 
@@ -1370,8 +1380,17 @@ func (api *API) userOIDC(rw http.ResponseWriter, r *http.Request) {
 		ok = false
 		emailSp := strings.Split(email, "@")
 		if len(emailSp) == 1 {
-			httpapi.Write(ctx, rw, http.StatusForbidden, codersdk.Response{
-				Message: fmt.Sprintf("Your email %q is not from an authorized domain! Please contact your administrator.", email),
+			site.RenderStaticErrorPage(rw, r, site.ErrorPageData{
+				Status:     http.StatusForbidden,
+				HideStatus: true,
+				Title:      "Unauthorized email",
+				Description: fmt.Sprintf(
+					"Your email %q is not from an authorized domain! Please contact your administrator.",
+					email,
+				),
+				Actions: []site.Action{
+					{URL: "/login", Text: "Back to login"},
+				},
 			})
 			return
 		}
@@ -1385,8 +1404,17 @@ func (api *API) userOIDC(rw http.ResponseWriter, r *http.Request) {
 			}
 		}
 		if !ok {
-			httpapi.Write(ctx, rw, http.StatusForbidden, codersdk.Response{
-				Message: fmt.Sprintf("Your email %q is not from an authorized domain! Please contact your administrator.", email),
+			site.RenderStaticErrorPage(rw, r, site.ErrorPageData{
+				Status:     http.StatusForbidden,
+				HideStatus: true,
+				Title:      "Unauthorized email",
+				Description: fmt.Sprintf(
+					"Your email %q is not from an authorized domain! Please contact your administrator.",
+					email,
+				),
+				Actions: []site.Action{
+					{URL: "/login", Text: "Back to login"},
+				},
 			})
 			return
 		}
@@ -1406,7 +1434,6 @@ func (api *API) userOIDC(rw http.ResponseWriter, r *http.Request) {
 	if ok {
 		picture, _ = pictureRaw.(string)
 	}
-
 	ctx = slog.With(ctx, slog.F("email", email), slog.F("username", username), slog.F("name", name))
 
 	user, link, err := findLinkedUser(ctx, api.Database, oidcLinkedID(idToken), email)