Fix webauthn regression and improve code (#25113)

Follow:

* #22697

There are some bugs in #22697:

* https://github.com/go-gitea/gitea/pull/22697#issuecomment-1577957966
* the webauthn failure message is never shown and causes console error
* The `document.getElementById('register-button')` and
`document.getElementById('login-button')` is wrong
    * there is no such element in code
    * it causes JS error when a browser doesn't provide webauthn
    * the end user can't see the real error message

These bugs are fixed in this PR.

Other changes:

* Use simple HTML/CSS layouts, no need to use too many `gt-` patches
* Make the webauthn page have correct "page-content" layout
* The "data-webauthn-error-msg" elements are only used to provide locale
texts, so move them into a single "gt-hidden", then no need to repeat a
lot of "gt-hidden" in code
* The `{{.CsrfTokenHtml}}`  is a no-op because there is no form
* Many `hideElem('#webauthn-error')` in code is no-op because the
`webauthn-error` already has "gt-hidden" by default
* Make the tests for "URLEncodedBase64" really test with concrete cases.


Screenshots:

* Error message when webauthn fails (before, there is no error message):

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/93cf9559-d93b-4f06-9d98-0f7032d9c65b)

</details>

* Error message when webauthn is unavailable 

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/ffc0fcd9-b93b-4418-979c-c89bb627aaf2)

</details>

1 files changed, 12 insertions, 3 deletions

diff --git a/web_src/js/utils.test.js b/web_src/js/utils.test.js
index cf73b63b99..812ef3bedf 100644
--- a/web_src/js/utils.test.js
+++ b/web_src/js/utils.test.js
@@ -133,8 +133,17 @@ test('toAbsoluteUrl', () => {
   expect(() => toAbsoluteUrl('path')).toThrowError('unsupported');
 });
 
+const uint8array = (s) => new TextEncoder().encode(s);
 test('encodeURLEncodedBase64, decodeURLEncodedBase64', () => {
-  expect(encodeURLEncodedBase64(decodeURLEncodedBase64('foo'))).toEqual('foo'); // No = padding
-  expect(encodeURLEncodedBase64(decodeURLEncodedBase64('a-minus'))).toEqual('a-minus');
-  expect(encodeURLEncodedBase64(decodeURLEncodedBase64('_underscorc'))).toEqual('_underscorc');
+  expect(encodeURLEncodedBase64(uint8array('AA?'))).toEqual('QUE_'); // standard base64: "QUE/"
+  expect(encodeURLEncodedBase64(uint8array('AA~'))).toEqual('QUF-'); // standard base64: "QUF+"
+
+  expect(decodeURLEncodedBase64('QUE/')).toEqual(uint8array('AA?'));
+  expect(decodeURLEncodedBase64('QUF+')).toEqual(uint8array('AA~'));
+  expect(decodeURLEncodedBase64('QUE_')).toEqual(uint8array('AA?'));
+  expect(decodeURLEncodedBase64('QUF-')).toEqual(uint8array('AA~'));
+
+  expect(encodeURLEncodedBase64(uint8array('a'))).toEqual('YQ'); // standard base64: "YQ=="
+  expect(decodeURLEncodedBase64('YQ')).toEqual(uint8array('a'));
+  expect(decodeURLEncodedBase64('YQ==')).toEqual(uint8array('a'));
 });