fix(frontend): use opaque background icons for PWA install

The manifest's largest, maskable icon (ms-icon-310x310) was transparent,
so installing the PWA produced a transparent app icon. Generate opaque
192/512 "any" + maskable icons from favicon-bg.png (solid #524B6B
background, maskable variants padded into the inner 80% safe zone) and
point the manifest at them instead of the transparent ms-icon entry.

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

frontend/static/manifest.webmanifest

@@ -35,21 +35,28 @@
       "type": "image/png"
     },
     {
-      "src": "/images/android-icon-192x192.png",
+      "src": "/images/pwa-192x192.png",
       "sizes": "192x192",
       "type": "image/png",
       "purpose": "any"
     },
     {
-      "src": "/images/apple-icon-180x180.png",
-      "sizes": "180x180",
-      "type": "image/png"
+      "src": "/images/pwa-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
     },
     {
-      "src": "/images/ms-icon-310x310.png",
-      "sizes": "310x310",
+      "src": "/images/pwa-maskable-192x192.png",
+      "sizes": "192x192",
       "type": "image/png",
-      "purpose": "any maskable"
+      "purpose": "maskable"
+    },
+    {
+      "src": "/images/pwa-maskable-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
     }
   ]
 }