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/build.gradle.kts

@@ -21,7 +21,7 @@ android {
         versionCode = 1
         versionName = "0.1.0"
         // Prefilled beacon config (dev tool — secret in the APK is fine).
-        buildConfigField("String", "BEACON_URL", "\"http://fmr-1.echo-lot.app:9099/beacon\"")
+        buildConfigField("String", "BEACON_URL", "\"http://89.185.109.150:443/beacon\"")
         buildConfigField("String", "BEACON_SECRET", "\"D4OmG5gGJsElqVVbtYIZbR\"")
     }
     buildTypes { release { isMinifyEnabled = false } }