feat: registration_enabled admin setting — hides signup surfaces, gates the API

One boolean (Security > Public Registration) switches the whole public
signup lifecycle: front page + login links, the logged-out sidebar's
Create-an-account (via a package-level templates hook so shared page
templates keep their signatures), GET /register -> 302 /login, and an
early 403 on POST /api/auth/register. First-user exception preserved:
zero admins keeps every route and link reachable for bootstrapping —
same 'users exist' reasoning as the setup gate. Admin user creation is
unaffected by design.

Live E2E verified: flag off hides all surfaces and blocks the POST
(403), flag on restores them (201); the admin UI renders the row
automatically.

templates/index.templ

@@ -1,6 +1,6 @@
 package templates
 
-templ Index(loggedIn bool) {
+templ Index(loggedIn bool, registrationEnabled bool) {
 	<!DOCTYPE html>
 	<html lang="en">
 		<head>
@@ -97,10 +97,12 @@ templ Index(loggedIn bool) {
 								</div>
 								<button type="submit" class="btn btn-primary w-full py-2.5">Sign In</button>
 							</form>
-							<p class="text-center text-sm mt-4" style="color: var(--text-secondary)">
-								No account?
-								<a href="/register" class="font-medium hover:underline" style="color: var(--accent)">Register</a>
-							</p>
+							if registrationEnabled {
+								<p class="text-center text-sm mt-4" style="color: var(--text-secondary)">
+									No account?
+									<a href="/register" class="font-medium hover:underline" style="color: var(--accent)">Register</a>
+								</p>
+							}
 						</div>
 						<div id="auth-result" class="mt-4 text-center text-sm"></div>
 					</div>