Theme-aware SVG favicon; SW cache v13

- Add sim-analyser-light.svg (#2563eb, light-theme accent); dark
  keeps the existing #3b82f6 variant.
- applyTheme() now swaps the favicon href alongside data-theme and
  theme-color, so the icon follows toggle click, saved theme on
  load, and OS-preference fallback (all route through applyTheme).
- Precache the new asset in the service worker.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simtrace-analyser-v12';
+const CACHE = 'simtrace-analyser-v13';
 
 const PRECACHE = [
     'index.html',
@@ -7,6 +7,7 @@ const PRECACHE = [
     'icon-512.png',
     'favicon.png',
     'sim-analyser.svg',
+    'sim-analyser-light.svg',
 ];
 
 self.addEventListener('install', e => {