style(settings): move tabs to a vertical column on the left

Wrapped the tab bar + body in a .settings-main flex row; tabs now stack
vertically on the left with a right border, body scrolls on the right. JS
unchanged (toggles .is-tab-hidden on sections regardless of layout).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

index.html

@@ -413,6 +413,7 @@
                 <svg><use href="#i-x"/></svg>
             </button>
         </div>
+        <div class="settings-main">
         <div class="settings-tabs" id="settingsTabs">
             <button class="settings-tab is-active" data-tab="general">Général</button>
             <button class="settings-tab" data-tab="security">Sécurité</button>
@@ -865,6 +866,7 @@
                 </div>
             </div>
         </div>
+        </div>
     </aside>
 
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