Fix MAC normalization in device_labels lookup; add label support to arrival/departure alerts

device_labels keys are normalized (no colons, uppercase) but known_devices keys
are colon-lowercase. device_labels.get(mac) always returned None.

Also extend label display to arrival and departure alerts — if a MAC has a
configured label, show that instead of vendor/hostname.

net_alerter/net_alerter.py

@@ -875,22 +875,22 @@ def fmt_duration(secs: float) -> str:
 
 
 def format_arrival(hostname: str, ip: str, vendor: str, mac: str) -> str:
-    """Format arrival alert message. Deduplicate IP if hostname equals IP."""
+    """Format arrival alert message. Uses friendly label if configured, else hostname/vendor."""
+    label = device_labels.get(_normalize_mac(mac))
+    if label:
+        return f"[NET] ARRIVED: {label} ({ip})"
     if hostname == ip:
-        # Hostname resolution failed, don't repeat IP
         return f"[NET] ARRIVED: {ip} [{vendor}] MAC:{mac}"
-    else:
-        # Hostname resolved successfully, show both
     return f"[NET] ARRIVED: {hostname} ({ip}) [{vendor}] MAC:{mac}"
 
 
 def format_departure(hostname: str, ip: str, vendor: str, mac: str, duration: str) -> str:
-    """Format departure alert message. Deduplicate IP if hostname equals IP."""
+    """Format departure alert message. Uses friendly label if configured, else hostname/vendor."""
+    label = device_labels.get(_normalize_mac(mac))
+    if label:
+        return f"[NET] DEPARTED: {label} ({ip}) — present {duration}"
     if hostname == ip:
-        # Hostname resolution failed, don't repeat IP
         return f"[NET] DEPARTED: {ip} [{vendor}] MAC:{mac} — present {duration}"
-    else:
-        # Hostname resolved successfully, show both
     return f"[NET] DEPARTED: {hostname} ({ip}) [{vendor}] MAC:{mac} — present {duration}"
 
 
@@ -916,7 +916,7 @@ def _update_occupancy_state_unlocked() -> None:
             present = []
             for mac, dev in known_devices.items():
                 if not dev.get('departing', False) and dev['ip'] not in infrastructure_ips:
-                    label = device_labels.get(mac) or dev.get('vendor') or mac[-8:]
+                    label = device_labels.get(_normalize_mac(mac)) or dev.get('vendor') or mac[-8:]
                     present.append(label)
             msg = f"[NET] Location: OCCUPIED ({', '.join(present)})"
         else: