Add Passkey login support (#31504)

closes #22015

After adding a passkey, you can now simply login with it directly by
clicking `Sign in with a passkey`.

![Screenshot from 2024-06-26
12-18-17](https://github.com/go-gitea/gitea/assets/6918444/079013c0-ed70-481c-8497-4427344bcdfc)

Note for testing. You need to run gitea using `https` to get the full
passkeys experience.

---------

Co-authored-by: silverwind <me@silverwind.io>

templates/user/auth/signin_inner.tmpl

@@ -9,6 +9,8 @@
 	{{end}}
 </h4>
 <div class="ui attached segment">
+	{{template "user/auth/webauthn_error" .}}
+
 	<form class="ui form tw-max-w-2xl tw-m-auto" action="{{.SignInLink}}" method="post">
 	{{.CsrfTokenHtml}}
 	<div class="required field {{if and (.Err_UserName) (or (not .LinkAccountMode) (and .LinkAccountMode .LinkAccountModeSignIn))}}error{{end}}">
@@ -49,6 +51,10 @@
 		</div>
 	{{end}}
 
+	<div class="field">
+		<a class="signin-passkey">{{ctx.Locale.Tr "auth.signin_passkey"}}</a>
+	</div>
+
 	{{if .OAuth2Providers}}
 	<div class="divider divider-text">
 		{{ctx.Locale.Tr "sign_in_or"}}