feat(settings): group settings into 4 tabs

Long settings panel → 4 tabs (Général / Sécurité / Account / Sync & Backup).
Each section (.slideover-field) is keyed by its label text to a tab via
SETTINGS_TAB_OF; applySettingsTab toggles .is-tab-hidden on the rest. No HTML
restructure (sections were already .slideover-field siblings), no dep. The
existing settings search composes: a live query suspends the tab filter so
cross-tab matches show, clearing it restores the active tab.

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

index.html

@@ -413,6 +413,12 @@
                 <svg><use href="#i-x"/></svg>
             </button>
         </div>
+        <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>
+            <button class="settings-tab" data-tab="account">Account</button>
+            <button class="settings-tab" data-tab="sync">Sync &amp; Backup</button>
+        </div>
         <div class="slideover-body" id="settingsBody">
             <div class="slideover-field">
                 <div class="slideover-field-label">Appearance</div>