fix(frontend): fit PWA icons into maskable safe zone

Android's adaptive-icon mask clipped the bamboo artwork and showed a
white backing on install. Fix the icon set so it survives masking.

- maskable icons: artwork fitted inside the 0.40W safe-zone circle,
  opaque full-bleed background (no transparency, so no white fill)
- any icons: rounded-corner tiles with transparent corners
- drop legacy transparent android-icon-* (RealFaviconGenerator) that
  Android picked up as adaptive icons and backed with white
- rename to icon-tile-*/icon-maskable-* so the manifest src changes
  and installed PWAs re-fetch the launcher icon

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

frontend/static/manifest.webmanifest

@@ -10,53 +10,28 @@
   "theme_color": "#312F45",
   "icons": [
     {
-      "src": "/images/android-icon-36x36.png",
-      "sizes": "36x36",
-      "type": "image/png"
-    },
-    {
-      "src": "/images/android-icon-48x48.png",
-      "sizes": "48x48",
-      "type": "image/png"
-    },
-    {
-      "src": "/images/android-icon-72x72.png",
-      "sizes": "72x72",
-      "type": "image/png"
-    },
-    {
-      "src": "/images/android-icon-96x96.png",
-      "sizes": "96x96",
-      "type": "image/png"
-    },
-    {
-      "src": "/images/android-icon-144x144.png",
-      "sizes": "144x144",
-      "type": "image/png"
-    },
-    {
-      "src": "/images/pwa-192x192.png",
+      "src": "/images/icon-tile-192x192.png",
       "sizes": "192x192",
       "type": "image/png",
       "purpose": "any"
     },
     {
-      "src": "/images/pwa-512x512.png",
+      "src": "/images/icon-tile-512x512.png",
       "sizes": "512x512",
       "type": "image/png",
       "purpose": "any"
     },
     {
-      "src": "/images/pwa-maskable-192x192.png",
+      "src": "/images/icon-maskable-192x192.png",
       "sizes": "192x192",
       "type": "image/png",
       "purpose": "maskable"
     },
     {
-      "src": "/images/pwa-maskable-512x512.png",
+      "src": "/images/icon-maskable-512x512.png",
       "sizes": "512x512",
       "type": "image/png",
       "purpose": "maskable"
     }
   ]
-}
+}