Add theme-adaptive screenshots to README and fix manifest icons

Replace old screenshots with new mobile/tablet variants in light and dark
mode. README now uses <picture> elements with prefers-color-scheme so
screenshots automatically match the viewer's GitHub theme. Split manifest
icon purpose field into separate "any" and "maskable" entries per PWA spec.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/manifest.json

@@ -13,13 +13,25 @@
       "src": "/icons/icon-192.png",
       "sizes": "192x192",
       "type": "image/png",
-      "purpose": "any maskable"
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
     },
     {
       "src": "/icons/icon-512.png",
       "sizes": "512x512",
       "type": "image/png",
-      "purpose": "any maskable"
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
     },
     {
       "src": "/icons/apple-touch-icon.png",