feat(pwa): add maskable icons for Android adaptive icons (N108)

The manifest had no purpose:"maskable" icon, so Android cropped a non-safe-zoned
icon (corners clipped / inconsistent shape). Added 192px + 512px maskable icons
(logo centered at ~62% on the app background_color #0a0a0a, inside the 80% safe
zone) generated from the existing logo, and registered them with
purpose:"maskable". They sit beside the existing android-chrome icons and use
the same manifest path convention, so they resolve identically to those.

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

public/manifest.json

@@ -54,6 +54,18 @@
       "src": "./res/android/android-chrome-512x512.png",
       "sizes": "512x512",
       "type": "image/png"
+    },
+    {
+      "src": "./res/android/maskable-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
+    {
+      "src": "./res/android/maskable-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
     }
   ],
   "categories": ["social", "communication", "productivity"],