fix: Remove maskable purpose from icons to prevent cropping

The "purpose": "any maskable" property caused Android to apply
adaptive icon treatment (circular/rounded square mask) to icons
that were not designed with safe zones for maskable display.

This resulted in the icon being incorrectly cropped on Android home screen.

Removed maskable purpose while keeping useful PWA enhancements:
- description field
- categories
- start_url and scope

Icons now display correctly without cropping.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/static/manifest.json

@@ -7,14 +7,12 @@
     {
       "src": "/static/images/android-chrome-192x192.png",
       "sizes": "192x192",
-      "type": "image/png",
-      "purpose": "any maskable"
+      "type": "image/png"
     },
     {
       "src": "/static/images/android-chrome-512x512.png",
       "sizes": "512x512",
-      "type": "image/png",
-      "purpose": "any maskable"
+      "type": "image/png"
     }
   ],
   "theme_color": "#0d6efd",