UI modernization: centralize SVG icons and adopt CSS variables

- Create web/utils/IconUtils.js with centralized SVG icon definitions
- Replace inline SVGs in linker.js with getSvgIcon() calls
- Refactor CSS to use CSS custom properties throughout
- Add renderProgressWithAction() helper for consistent progress UI
- Add renderOnlineSearchResultCard() helper for search results
- Add MLToggleHidden() utility for show/hide toggle
- Replace hardcoded spacing, fonts, colors with CSS variables
- Standardize markup with reusable CSS classes

web/css/css-variables.css

@@ -67,6 +67,7 @@
     --ml-spacing-4: 4px;
     --ml-spacing-5: 5px;
     --ml-spacing-6: 6px;
+    --ml-spacing-7: 7px;
     --ml-spacing-8: 8px;
     --ml-spacing-10: 10px;
     --ml-spacing-12: 12px;