fix: web/ quick wins — CSS cleanup, ARIA tabs, and file header comments

CSS:
- Remove duplicate li::before block in app-search.css
- Fix typo: search-reuslt → search-result in app-search.css
- Remove commented-out CSS rules in app-flash.css
- Add descriptive header comment to all 27 CSS component files

JS:
- Complete WAI-ARIA Tabs pattern: generate IDs, add aria-controls on
  tab buttons and aria-labelledby on tab panels (app-tabs.js)
- Add JSDoc header comments to ~33 JS files (core + components)
- Add explanatory block comment to app-boot.js (why classic IIFE)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/js/components/app-theme-toggle.js

@@ -1,3 +1,6 @@
+/**
+ * Theme switcher — light/dark toggle and menu with server-side persistence.
+ */
 import { optionalEl, warnOnce } from '../core/app-dom.js';
 
 const setTheme = (theme) => {