Fix DHCP client unable to get IP and config not persisting

- Fixed verifyAssignment being too strict for new clients
- Fixed parseRequestedIP string conversion bug
- Fixed response sent to 0.0.0.0 instead of broadcast address
- Added SO_BROADCAST support for UDP socket
- Fixed session persistence after page refresh (localStorage)
- Added in-memory session store for auth middleware
- Added config reloader so DHCP server picks up web UI changes dynamically

configs/config.json

@@ -0,0 +1,39 @@
+{
+  "dhcp": {
+    "enabled": true,
+    "interface": "eth0",
+    "network": "192.168.1.0",
+    "netmask": "255.255.255.0",
+    "gateway": "192.168.1.1",
+    "dns_servers": ["192.168.1.1", "114.114.114.114", "8.8.8.8"],
+    "ntp_servers": ["ntp.aliyun.com"],
+    "broadcast_address": "192.168.1.255",
+    "lease_time": 86400,
+    "ip_pool_start": "192.168.1.100",
+    "ip_pool_end": "192.168.1.200",
+    "domain_name": "local",
+    "excluded_ips": ["192.168.1.1", "192.168.1.2", "192.168.1.3"]
+  },
+  "dns": {
+    "enabled": true,
+    "listen_addr": "0.0.0.0",
+    "listen_port": 53,
+    "upstream": ["8.8.8.8", "1.1.1.1", "114.114.114.114"],
+    "cache_size": 1000,
+    "cache_ttl": 300,
+    "recursion": true,
+    "allow_query": ["any"],
+    "dnssec_validation": false
+  },
+  "web": {
+    "host": "0.0.0.0",
+    "port": 8080,
+    "session_key": "change-this-to-a-random-secret-key",
+    "enable_https": false,
+    "https_cert": "",
+    "https_key": ""
+  },
+  "database": {
+    "path": "data/dhcp-dns.db"
+  }
+}