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"

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

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="wrap_content"
+    android:orientation="vertical"
+    android:padding="20dp">
+
+    <TextView
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="@string/msg_overwrite_warning"
+        android:textSize="14sp"
+        android:paddingBottom="16dp" />
+
+    <CheckBox
+        android:id="@+id/confirmCheckBox"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="@string/checkbox_confirm_overwrite" />
+
+</LinearLayout>

android/app/src/main/res/values/strings.xml

@@ -15,6 +15,7 @@
     <string name="status_bluetooth_disabled">Bluetooth not enabled</string>
     <string name="status_scanning">Scanning…</string>
     <string name="status_device_not_found">Device not found</string>
+    <string name="status_wrong_device">Verkeerd apparaat — opnieuw scannen…</string>
     <string name="button_retry_scan">Opnieuw scannen</string>
 
     <!-- MainActivity UI -->
@@ -29,6 +30,20 @@
     <string name="key_exchange_failed">Sleutel uitwisselen mislukt - probeer opnieuw</string>
     <string name="flash_failed">Flashen mislukt: %s</string>
 
+    <!-- Key conflict dialog -->
+    <string name="title_key_conflict">Sleutels komen niet overeen</string>
+    <string name="msg_key_conflict">De ESP (SylDa_Server) heeft andere sleutels dan deze app. Wat wil je doen?</string>
+    <string name="button_keep_esp_keys">ESP sleutels overnemen</string>
+    <string name="button_overwrite_esp">App sleutels overzetten</string>
+    <string name="button_confirm">Bevestigen</string>
+
+    <!-- Overwrite confirmation dialog -->
+    <string name="title_overwrite_confirm">ESP sleutels overschrijven?</string>
+    <string name="msg_overwrite_warning">Dit vervangt de sleutels van de ESP. De ESP kan daarna niet meer met de oorspronkelijke app communiceren.</string>
+    <string name="checkbox_confirm_overwrite">Ik weet het zeker</string>
+    <string name="title_overwriting_keys">Sleutels overschrijven</string>
+    <string name="msg_writing_keys">Sleutels schrijven naar ESP…</string>
+
     <!-- SettingsActivity dialogs -->
     <string name="title_key_exchange">Sleutel Uitwisselen</string>
     <string name="title_firmware_flash">Firmware Flashen</string>