standardise buttons: .btn base class (border-radius 10px, padding var(--space-xs))

app/templates/admin/login.php

@@ -11,7 +11,7 @@
                     <input type="password" id="password" name="password" required autofocus>
                 </div>
                 <div class="admin-form-footer">
-                    <button type="submit" class="admin-btn">Se connecter</button>
+                    <button type="submit" class="btn btn--primary">Se connecter</button>
                 </div>
             </form>
         </div>