dhcp: track pending config changes with hash, update UI button

lib/state.py

@@ -5,6 +5,8 @@ state instead of invoking subprocesses on every request.
 """
 
 import contextlib
+import hashlib
+import json
 import logging
 import os
 from copy import deepcopy
@@ -588,12 +590,23 @@ def _collect_dnsmasq() -> dict[str, Any]:
     # Check config file on disk
     conf_exists = Path(DNSMASQ_CONF).is_file()
 
+    # Check if JSON config has changed since last apply
+    _APPLY_HASH_KEY = "_last_applied_hash"
+    pending_changes = True
+    if _APPLY_HASH_KEY in cfg:
+        clean = {k: v for k, v in cfg.items() if k != _APPLY_HASH_KEY}
+        current_hash = hashlib.sha256(
+            json.dumps(clean, sort_keys=True).encode()
+        ).hexdigest()
+        pending_changes = cfg[_APPLY_HASH_KEY] != current_hash
+
     return {
         "config": cfg,
         "status": {
             "service_active": service_active,
             "config_file_exists": conf_exists,
             "active_leases": len(leases),
+            "pending_changes": pending_changes,
         },
         "leases": leases,
         "timestamp": _now_iso(),