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/index.html

@@ -5,7 +5,8 @@
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>OTAMan</title>
 <link rel="stylesheet" href="style.css">
-<link rel="icon" type="image/png" href="sim.png">
+<link rel="icon" type="image/svg+xml" href="favicon.svg">
+<link rel="icon" type="image/png" href="sim.png" sizes="462x462">
 <link rel="manifest" href="manifest.json">
 <meta name="apple-mobile-web-app-capable" content="yes">
 <link rel="apple-touch-icon" href="icon-192.png">