Add post-installation redirect based on admin account status (#34493)

This PR adds a feature to direct users to appropriate pages after system
installation:
- If no admin credentials were provided during installation, redirect to
the registration page with a prominent notice about creating the first
administrative account
- If admin credentials were already set, redirect directly to the login
page


![4d396ad132d9b57fc4f45a62117177f1](https://github.com/user-attachments/assets/3a5d8700-9194-4d3b-a862-e64c8c347932)

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/post-install.tmpl

@@ -4,7 +4,7 @@
 		<!-- the "cup" has a handler, so move it a little leftward to make it visually in the center -->
 		<div class="tw-ml-[-30px]"><img width="160" src="{{AssetUrlPrefix}}/img/loading.png" alt aria-hidden="true"></div>
 		<div class="tw-my-[2em] tw-text-[18px]">
-			<a id="goto-user-login" href="{{AppSubUrl}}/user/login">{{ctx.Locale.Tr "install.installing_desc"}}</a>
+			<a id="goto-after-install" href="{{AppSubUrl}}{{Iif .IsAccountCreated "/user/login" "/user/sign_up"}}">{{ctx.Locale.Tr "install.installing_desc"}}</a>
 		</div>
 	</div>
 </div>

templates/user/auth/signup_inner.tmpl

@@ -7,6 +7,9 @@
 		{{end}}
 	</h4>
 	<div class="ui attached segment">
+		{{if .IsFirstTimeRegistration}}
+			<p>{{ctx.Locale.Tr "auth.sign_up_tip"}}</p>
+		{{end}}
 		<form class="ui form" action="{{.SignUpLink}}" method="post">
 			{{.CsrfTokenHtml}}
 			{{if or (not .LinkAccountMode) (and .LinkAccountMode .LinkAccountModeRegister)}}