chore: ui polish — extract inline styles to CSS classes, add header art

- Move inline style.cssText to semantic classes (cal-legend, cal-header-cell,
  cal-day-tag-label, cal-btn-row) in calendar.js
- Buttons "FT"/"BU" → "Feiertag"/"Betriebsurl." for clarity
- Decorative turntable/mixer SVG header art in index.html
- style.css build-out for the new classes
- Initial README content
- Dev port 3000 → 3001

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

package.json

@@ -4,7 +4,7 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "start": "npx serve . -p 3000",
+    "start": "npx serve . -p 3001",
     "test": "node tests/algorithm.test.mjs"
   }
 }