tools: beacon fixes — cleartext, multi-device, validated-net POST, status URL

Debugging against a real restricted LAN surfaced three fixes:
- Android blocks app cleartext HTTP by default (targetSdk 36) — the port
  discovery + reachability were fine (phone curl reached fmr), only the
  app POST was denied. Added usesCleartextTraffic for this dev tool.
- Multi-device: report + receiver are keyed by device (Build.MODEL) so a
  phone and tablet don't clobber each other; connector connects each.
- POST over a VALIDATED internet network (prefer cellular) since the
  wireless-debug wifi is often a restricted LAN.
- Status/notification now show the target beacon URL + which network, per
  the request to surface what it's connecting to.
Verified live: beacon tracks the (frequently rotating) port via mDNS and
self-reports the current endpoint within seconds.

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

echolot-app/adb-beacon/src/main/AndroidManifest.xml

@@ -13,6 +13,7 @@
     <application
         android:allowBackup="false"
         android:label="Echolot ADB Beacon"
+        android:usesCleartextTraffic="true"
         android:theme="@android:style/Theme.Material.Light">
 
         <activity android:name=".MainActivity" android:exported="true">