fix(web): split maskable icon into separate entries for type safety

Next.js MetadataRoute.Manifest types only accept single-value purpose
fields, not space-separated combinations like "any maskable".

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

apps/web/app/manifest.ts

@@ -22,7 +22,13 @@ export default function manifest(): MetadataRoute.Manifest {
         src: "/icon-512x512.png",
         sizes: "512x512",
         type: "image/png",
-        purpose: "any maskable",
+        purpose: "any",
+      },
+      {
+        src: "/icon-512x512.png",
+        sizes: "512x512",
+        type: "image/png",
+        purpose: "maskable",
       },
     ],
   };