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/index.html

@@ -7,7 +7,8 @@
   <title>Calendarr</title>
   <link rel="icon" type="image/svg+xml" href="/static/favicon.svg" />
   <link rel="manifest" href="/manifest.json" />
-  <meta name="theme-color" content="#4285f4" />
+  <meta name="theme-color" content="#16713d" />
+  <meta name="mobile-web-app-capable" content="yes" />
   <meta name="apple-mobile-web-app-capable" content="yes" />
   <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
   <meta name="apple-mobile-web-app-title" content="Calendarr" />