fix: rename data-panel to data-tab on settings panels to match switchSettingsTab()

switchSettingsTab() in app.js reads panel.dataset.tab (data-tab attribute).
The four .settings-panel elements in index.html were using data-panel instead,
causing all panels to get hidden on every tab click — the settings dialog rendered
as a blank content area after any tab interaction.

Fix: Change data-panel="..." to data-tab="..." on all four panel divs.

Tests added:
- test_html_settings_panels_use_data_tab: verifies each .settings-panel uses
  data-tab and not data-panel
- test_html_settings_tab_panel_data_tab_alignment: cross-checks that every tab
  button data-tab value has a matching .settings-panel data-tab value, preventing
  this class of HTML/JS attribute mismatch from recurring

muxplex/frontend/index.html

@@ -80,7 +80,7 @@
         <button class="settings-tab" data-tab="new-session">New Session</button>
       </nav>
       <div class="settings-content">
-        <div class="settings-panel" data-panel="display">
+        <div class="settings-panel" data-tab="display">
           <div class="settings-field">
             <label class="settings-label" for="setting-font-size">Font Size</label>
             <select id="setting-font-size" class="settings-select">
@@ -111,9 +111,9 @@
             </select>
           </div>
         </div>
-        <div class="settings-panel hidden" data-panel="sessions"></div>
-        <div class="settings-panel hidden" data-panel="notifications"></div>
-        <div class="settings-panel hidden" data-panel="new-session"></div>
+        <div class="settings-panel hidden" data-tab="sessions"></div>
+        <div class="settings-panel hidden" data-tab="notifications"></div>
+        <div class="settings-panel hidden" data-tab="new-session"></div>
       </div>
     </div>
   </dialog>