feat: add device UUID pairing — unique identity per ESP-phone pair

- Firmware: auto-generate 16-byte UUID on first boot (NVS), new READ characteristic
- BLE name: AtomS3R-Relay → SylDa_Server
- KEY? now returns DEVICE_ID + HMAC_KEY, SET_ID command for UUID provisioning
- App: UUID+HMAC loaded from prefs, scan by service UUID, verify UUID after connect
- UUID mismatch → disconnect + skip address + rescan
- Key conflict dialog: keep ESP keys, overwrite ESP (with checkbox confirm), or cancel
- NO_KEY with existing app keys: send existing pair instead of generating new
- Added design spec doc

android/app/src/main/res/layout/activity_main.xml

@@ -13,14 +13,31 @@
         android:background="?android:attr/selectableItemBackgroundBorderless"
         android:contentDescription="Instellingen" />
 
-    <Button
-        android:id="@+id/relayButton"
-        android:layout_width="200dp"
-        android:layout_height="200dp"
+    <LinearLayout
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
         android:layout_gravity="center"
-        android:text="HOLD"
-        android:textSize="24sp"
-        android:enabled="false" />
+        android:orientation="horizontal"
+        android:gravity="center">
+
+        <Button
+            android:id="@+id/relay1Button"
+            android:layout_width="150dp"
+            android:layout_height="200dp"
+            android:text="1"
+            android:textSize="24sp"
+            android:enabled="false" />
+
+        <Button
+            android:id="@+id/relay2Button"
+            android:layout_width="150dp"
+            android:layout_height="200dp"
+            android:layout_marginStart="16dp"
+            android:text="2"
+            android:textSize="24sp"
+            android:enabled="false" />
+
+    </LinearLayout>
 
     <TextView
         android:id="@+id/statusText"