Add UI theme settings and experimental dark theme support

- Introduced a new "Appearance" settings section in the settings modal to allow users to select between "Light" and "Dark (Experimental)" themes.
- Updated the application initialization to apply the saved UI theme immediately on page load.
- Enhanced the settings UI to load and save the selected theme.
- Implemented CSS variables for the new experimental dark theme, including color adjustments for various UI elements.
- Updated existing styles to support theme switching, ensuring proper color contrast and readability in both light and dark modes.

src/js/user-settings.js

@@ -19,6 +19,10 @@ const DEFAULT_SETTINGS = {
         renderDebounce: 1500       // 300-3000ms - delay before visualization renders
     },
 
+    ui: {
+        theme: 'light'             // 'light' | 'experimental'
+    },
+
     formatting: {
         dateFormat: 'smart',       // 'smart' | 'locale' | 'iso' | 'custom'
         customDateFormat: 'yyyy-MM-dd HH:mm'  // Used when dateFormat === 'custom'
@@ -284,5 +288,12 @@ function validateSetting(path, value) {
         }
     }
 
+    if (path === 'ui.theme') {
+        const validThemes = ['light', 'experimental'];
+        if (!validThemes.includes(value)) {
+            errors.push('Invalid theme value');
+        }
+    }
+
     return errors;
 }