feat(web): branded green PWA icons + maskable, installable app polish

Replace the stale blue calendar icon with the real green Calendarr app
icon (matching iOS/Android) for the installable web app:
- icon-192/512.png regenerated from the branded 1024 app icon
- add dedicated maskable icons (192/512) with brand-green safe-zone
  padding so Android/Chrome render a proper adaptive icon (no white box)
- manifest: maskable purpose entries, id, description, lang/dir,
  categories; theme_color -> brand green (#16713d)
- index.html: theme-color -> green, add modern mobile-web-app-capable
- favicon.svg + icon.svg recolored to brand green
Bump APP_VERSION v89 -> v90.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/icons/icon.svg

@@ -1,5 +1,5 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
-  <rect width="512" height="512" fill="#4285f4"/>
+  <rect width="512" height="512" fill="#16713d"/>
   <g fill="none" stroke="#ffffff" stroke-width="23" stroke-linecap="round" stroke-linejoin="round">
     <rect x="102" y="154" width="307" height="266" rx="26"/>
     <line x1="102" y1="234" x2="409" y2="234"/>