Implement better auth flow with direct Authentik redirects

- Create custom /login and /logout endpoints that bypass Auth.js default pages
- Use auto-submitting forms to POST to Auth.js with proper form data
- Update UserHeader links to use new custom endpoints (/login, /logout)
- Remove old login/logout page server files that are no longer needed
- Login flow: /login → auto-submit form → /auth/signin/authentik → Authentik
- Logout flow: /logout → auto-submit form → /auth/signout → Authentik logout
- Provides seamless user experience with loading spinners during redirects
- Maintains all Auth.js security features and session management
- Eliminates intermediate Auth.js pages for cleaner auth flow

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/lib/components/UserHeader.svelte

@@ -139,10 +139,10 @@ h2 + p{
 			<p>({user.nickname})</p>
 			<ul>
 				<li><a href="https://sso.bocken.org/if/user/#/settings" >Einstellungen</a></li>
-				<li><a href="/auth/signout" >Log Out</a></li>
+				<li><a href="/logout" >Log Out</a></li>
 			</ul>
 		</div>
 	</button>
 {:else}
-	<a class=entry href=/auth/signin>Log In</a>
+	<a class=entry href=/login>Log In</a>
 {/if}