Theme-adaptive favicon: transparent PNG + prefers-color-scheme SVG

- sim.png regenerated from sim.svg with true transparency (was flattened
  onto opaque white; favicon showed a white box)
- new favicon.svg: same glyph, fill moved to presentation attribute +
  embedded @media (prefers-color-scheme: dark) -> white glyph on dark
  browser themes (interface sim.svg untouched, keeps dark:invert)
- index.html: dual icon links (svg for modern browsers, png Safari fallback)
- sw.js: precache favicon.svg, cache bump otaman-v11 -> otaman-v12

frontend/sw.js

@@ -1,10 +1,11 @@
-const CACHE = 'otaman-v11';
+const CACHE = 'otaman-v12';
 const URLS = [
   'index.html',
   'help.html',
   'help-ru.html',
   'style.css',
   'sim.png',
+  'favicon.svg',
   'icon-192.png',
   'icon-512.png',
   'manifest.json',