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/manifest.json

@@ -1,12 +1,17 @@
 {
+  "id": "/",
   "name": "Calendarr",
   "short_name": "Calendarr",
+  "description": "Dein privater, selbst gehosteter Kalender.",
+  "lang": "de",
+  "dir": "ltr",
   "start_url": "/",
   "scope": "/",
   "display": "standalone",
   "orientation": "any",
   "background_color": "#0e0e14",
-  "theme_color": "#4285f4",
+  "theme_color": "#16713d",
+  "categories": ["productivity", "utilities"],
   "icons": [
     {
       "src": "/icons/icon-192.png",
@@ -21,10 +26,16 @@
       "purpose": "any"
     },
     {
-      "src": "/icons/icon.svg",
-      "sizes": "any",
-      "type": "image/svg+xml",
-      "purpose": "any"
+      "src": "/icons/icon-maskable-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
+    {
+      "src": "/icons/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
     }
   ]
 }