station: Surface signal, traffic and connection time in API and UI

Hostapd already parsed signal strength and rx/tx counters but the
station -> ConnectedDevice conversion threw them away. Add signalDbm,
rxBytes, txBytes and connectedAt to the OpenAPI schema and the
ConnectedDevice model, and centralise the conversion in
station.ToConnectedDevice so handlers, the periodic refresh and the
event callbacks all serialise the same shape.

Two follow-on bugs surfaced while wiring this up:

- The hostapd backend only stored station entries on first contact.
  Subsequent polls were dropped, so signal and byte counters never
  refreshed. Reconcile updates in checkStationChanges.
- ConnectedAt was reset to time.Now() on every conversion. Track
  FirstSeen on HostapdStation when the station joins, and preserve
  the timestamp across periodic refreshes in app.go so the UI's
  "connected since" badge is stable.

Frontend gains a metrics row on each device card with signal bars,
total traffic and a live duration. Falls back gracefully when a
backend (DHCP, ARP) doesn't expose these fields.

openapi.yaml

@@ -418,7 +418,7 @@ components:
       properties:
         name:
           type: string
-          description: Device hostname
+          description: Device hostname (may be empty when DHCP did not provide one)
           example: "iPhone-12"
         type:
           type: string
@@ -440,6 +440,30 @@ components:
           description: Assigned IP address
           pattern: '^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$'
           example: "192.168.1.100"
+        vendor:
+          type: string
+          description: Hardware vendor inferred from the MAC OUI prefix (empty when unknown)
+          example: "Apple"
+        signalDbm:
+          type: integer
+          format: int32
+          description: Wi-Fi signal strength in dBm (0 when unavailable, e.g. for non-hostapd backends)
+          example: -54
+        rxBytes:
+          type: integer
+          format: int64
+          description: Total bytes received from the device since connection (0 when unavailable)
+          example: 1048576
+        txBytes:
+          type: integer
+          format: int64
+          description: Total bytes sent to the device since connection (0 when unavailable)
+          example: 524288
+        connectedAt:
+          type: string
+          format: date-time
+          description: Timestamp when the station first appeared (best effort)
+          example: "2026-05-01T08:30:00Z"
       required:
         - name
         - type