Add BLE security: Just Works pairing + HMAC-SHA256 authentication

Two-layer security for the BLE relay:

1. BLE Just Works pairing (link-layer encryption)
   - ESP32 requires bonding before accepting writes
   - Characteristic permissions set to ESP_GATT_PERM_WRITE_ENCRYPTED
   - IO capability NO_INPUT_NO_OUTPUT (no passkey needed)
   - LE Secure Connections with bonding (ESP_LE_AUTH_REQ_SC_BOND)

2. HMAC-SHA256 authentication (application-level)
   - 32-byte shared secret in src/hmac_key.h (generated, gitignored)
   - Each command signed: HMAC-SHA256(key, command || sequence)
   - 37-byte packet format: [cmd:1][seq:4][hmac:32]
   - Sequence counter prevents replay (strictly increasing, starts at 1)
   - mbedtls on ESP, javax.crypto.Mac on Android

Key management:
- tools/generate_hmac_key.py generates random key + writes src/hmac_key.h
- ESP prints hex key to serial 5 seconds after boot
- Android app: SettingsActivity with eye-toggle password field
- Key stored in SharedPreferences, supports multiple ESP modules

Other changes:
- Android app: bonding flow (createBond -> BOND_BONDED -> discover)
- Android app: MTU negotiation (request 256)
- Android app: settings button (gear icon) in main screen
- Fix: sequence counter starts at 1 to prevent first-packet rejection
- Updated README with full security documentation

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

@@ -6,6 +6,16 @@
     android:gravity="center"
     android:padding="32dp">
 
+    <ImageButton
+        android:id="@+id/settingsButton"
+        android:layout_width="48dp"
+        android:layout_height="48dp"
+        android:layout_gravity="end"
+        android:src="@drawable/ic_settings"
+        android:background="?android:attr/selectableItemBackgroundBorderless"
+        android:contentDescription="Instellingen"
+        android:layout_marginBottom="16dp" />
+
     <TextView
         android:id="@+id/statusText"
         android:layout_width="wrap_content"