Reorganize repository into monorepo structure

Restructured the project to support multiple frontend applications:
- Move web app to web/ directory
- Create pezkuwi-sdk-ui/ for Polkadot SDK clone (planned)
- Create mobile/ directory for mobile app development
- Add shared/ directory with common utilities, types, and blockchain code
- Update README.md with comprehensive documentation
- Remove obsolete DKSweb/ directory

This monorepo structure enables better code sharing and organized
development across web, mobile, and SDK UI projects.

web/src/pages/NotFound.tsx

@@ -0,0 +1,27 @@
+import { useLocation } from "react-router-dom";
+import { useEffect } from "react";
+
+const NotFound = () => {
+  const location = useLocation();
+
+  useEffect(() => {
+    console.error(
+      "404 Error: User attempted to access non-existent route:",
+      location.pathname
+    );
+  }, [location.pathname]);
+
+  return (
+    <div className="min-h-screen flex items-center justify-center bg-background">
+      <div className="text-center p-8 rounded-lg border border-border bg-card shadow-md animate-slide-in">
+        <h1 className="text-5xl font-bold mb-6 text-primary">404</h1>
+        <p className="text-xl text-card-foreground mb-6">Page not found</p>
+        <a href="/" className="text-primary hover:text-primary/80 underline transition-colors">
+          Return to Home
+        </a>
+      </div>
+    </div>
+  );
+};
+
+export default NotFound;