feat: add visual customization presets to Settings > Appearance tab

Add style presets (font + color combinations) and background effect
presets as a new Appearance tab in the settings modal. Style presets
include Default, Midnight, Ember, Arctic, Terminal, and Neon — each
with curated dark/light theme colors and Google Font pairings.
Background effects (Dot Grid, Gradient Mesh, Scanlines, Particles)
use a dedicated overlay div alongside the existing WebGL Noise Field.
All choices persist to localStorage and restore on page load.

server/src/wled_controller/templates/modals/settings.html

@@ -1,6 +1,6 @@
 <!-- Settings Modal -->
 <div id="settings-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="settings-modal-title">
-    <div class="modal-content" style="max-width: 480px;">
+    <div class="modal-content" style="max-width: 520px;">
         <div class="modal-header">
             <h2 id="settings-modal-title" data-i18n="settings.title">Settings</h2>
             <button class="modal-close-btn" onclick="closeSettingsModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
@@ -11,6 +11,7 @@
             <button class="settings-tab-btn active" data-settings-tab="general" onclick="switchSettingsTab('general')" data-i18n="settings.tab.general">General</button>
             <button class="settings-tab-btn" data-settings-tab="backup" onclick="switchSettingsTab('backup')" data-i18n="settings.tab.backup">Backup</button>
             <button class="settings-tab-btn" data-settings-tab="mqtt" onclick="switchSettingsTab('mqtt')" data-i18n="settings.tab.mqtt">MQTT</button>
+            <button class="settings-tab-btn" data-settings-tab="appearance" onclick="switchSettingsTab('appearance')" data-i18n="settings.tab.appearance">Appearance</button>
         </div>
 
         <div class="modal-body">
@@ -233,6 +234,11 @@
                 </form>
             </div>
 
+            <!-- ═══ Appearance tab ═══ -->
+            <div id="settings-panel-appearance" class="settings-panel">
+                <!-- Rendered dynamically by renderAppearanceTab() -->
+            </div>
+
             <div id="settings-error" class="error-message" style="display:none;"></div>
         </div>
         <div class="modal-footer">