fix: change OAuth authentication link to open in same tab

Change target="_blank" to target="_self" in auth-url.html template to improve
user experience. When users click "Authenticate with Trakt.tv", the link now
opens in the same tab instead of a new window, allowing them to naturally
return to the application after OAuth callback completion.

Author: JohanDevl

web/templates/auth-url.html

@@ -47,7 +47,7 @@ <h1>🔗 OAuth Authentication</h1>
         </div>
 
         <div class="auth-action">
-            <a href="{{.AuthURL}}" target="_blank" class="btn btn-primary btn-large">
+            <a href="{{.AuthURL}}" target="_self" class="btn btn-primary btn-large">
                 🔗 Authenticate with Trakt.tv
             </a>
         </div>