Fix WebTransport server implementation and add test client

Server fixes:
- Move H3Connection initialization to ProtocolNegotiated event (matches official aioquic pattern)
- Fix datagram routing to use session_id instead of flow_id
- Add max_datagram_frame_size=65536 to enable QUIC datagrams
- Fix send_datagram() to use keyword arguments
- Add certificate chain handling for Let's Encrypt
- Add no-cache headers to static server

Command-line improvements:
- Move settings from environment variables to argparse
- Add comprehensive CLI arguments with defaults
- Default mode=wt, cert=cert.pem, key=key.pem

Test clients:
- Add test_webtransport_client.py - Python WebTransport client that successfully connects
- Add test_http3.py - Basic HTTP/3 connectivity test

Client updates:
- Auto-configure server URL and certificate hash from /cert-hash.json
- Add ES6 module support

Status:
✅ Python WebTransport client works perfectly
✅ Server properly handles WebTransport connections and datagrams
❌ Chrome fails due to cached QUIC state (QUIC_IETF_GQUIC_ERROR_MISSING)
🔍 Firefox sends packets but fails differently - to be debugged next session

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

test_http3.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+"""Simple HTTP/3 client to test the server."""
+
+import asyncio
+import logging
+from aioquic.asyncio import connect
+from aioquic.quic.configuration import QuicConfiguration
+
+logging.basicConfig(level=logging.DEBUG)
+
+async def test_http3(url: str):
+    """Test HTTP/3 connection to the server."""
+    print(f"Testing HTTP/3 connection to: {url}")
+
+    # Parse URL
+    if url.startswith("https://"):
+        url = url[8:]
+
+    host, port = url.split(":")
+    port = int(port.rstrip("/"))
+
+    print(f"Connecting to {host}:{port}...")
+
+    # Create QUIC configuration
+    configuration = QuicConfiguration(
+        is_client=True,
+        alpn_protocols=["h3"],
+        verify_mode=0  # Skip certificate verification for testing
+    )
+
+    try:
+        async with connect(
+            host,
+            port,
+            configuration=configuration,
+        ) as protocol:
+            print(f"✓ QUIC connection established!")
+            print(f"  ALPN protocol: {protocol._quic.tls.alpn_negotiated}")
+            print(f"  Remote address: {protocol._quic.remote_address}")
+
+            # Just test the connection, don't send HTTP requests yet
+            await asyncio.sleep(1)
+
+            print("✓ Connection successful!")
+            return True
+
+    except Exception as e:
+        print(f"✗ Connection failed: {e}")
+        import traceback
+        traceback.print_exc()
+        return False
+
+if __name__ == "__main__":
+    import sys
+
+    url = sys.argv[1] if len(sys.argv) > 1 else "https://127.0.0.1:4433"
+
+    success = asyncio.run(test_http3(url))
+    sys.exit(0 if success else 1)