Add 404 page and Web UI section to README

- Add styled 404 page for HTML requests (returns JSON for API calls)
- Add Web UI section to README documenting available routes

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

server.py

@@ -89,6 +89,23 @@ app = FastAPI(
 )
 
 
+@app.exception_handler(404)
+async def not_found_handler(request: Request, exc):
+    """Custom 404 page."""
+    accept = request.headers.get("accept", "")
+    if "text/html" in accept and "application/json" not in accept:
+        content = '''
+        <div class="text-center py-16">
+            <h2 class="text-6xl font-bold text-gray-600 mb-4">404</h2>
+            <p class="text-xl text-gray-400 mb-8">Page not found</p>
+            <a href="/" class="text-blue-400 hover:text-blue-300">Go to home page</a>
+        </div>
+        '''
+        username = get_user_from_cookie(request)
+        return HTMLResponse(base_html("Not Found", content, username), status_code=404)
+    return JSONResponse({"detail": "Not found"}, status_code=404)
+
+
 # ============ Data Models ============
 
 class Asset(BaseModel):