cockpit: unified Clients card (Wi-Fi + wired via bridge FDB)

New "Clients" card lists everything on the van LAN in one table — hostname,
IP, MAC, connection (5GHz / 2.4GHz / LAN eth0 / LAN USB), signal, TX rate.
Wi-Fi rows come from the hostapd station dumps as before; wired rows from
learned bridge-FDB entries on the LAN ports (the port's own MAC is
"permanent", and FDB duplicates entries per vlan — both filtered), enriched
with IP/hostname from dnsmasq leases + neighbor table.

The per-band station tables move out of the Access Points card (which keeps
its status line + client count), and the WAN/Uplinks table now hides
unmanaged devices so the networkd-owned LAN ports don't show up as ghost
WAN rows.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cockpit/vanrouter/index.html

@@ -20,6 +20,11 @@
             <div id="ap"></div>
         </div>
 
+        <div class="card">
+            <h3>Clients</h3>
+            <div id="clients"></div>
+        </div>
+
         <div class="card">
             <h3>Temperatures</h3>
             <div id="thermal"></div>