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/favicon.svg

@@ -1,4 +1,4 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#4285f4" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#2ea05a" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
   <rect x="3" y="4" width="18" height="18" rx="2"/>
   <line x1="16" y1="2" x2="16" y2="6"/>
   <line x1="8" y1="2" x2="8" y2="6"/>

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"/>

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" />

frontend/js/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v89';
+export const APP_VERSION = 'v90';

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"
     }
   ]
 }