Add memory leak detection with LeakCanary

[callebtc]

Description:
The app has potential memory leaks (Handler cleanup, resource management) but lacks automated detection during development.

Benefits of LeakCanary:

1. Automatic memory leak detection
2. Detailed leak traces and analysis
3. Helps identify Handler, Activity, and resource leaks
4. Essential for POS apps that run continuously

Fix Instructions:

1. Add LeakCanary dependency to build.gradle:

dependencies {
    // Debug builds only
    debugImplementation "com.squareup.leakcanary:leakcanary-android:2.12"
}

2. LeakCanary automatically initializes, no code changes needed

3. Test scenarios prone to leaks:

   • Rapidly opening/closing activities
   • NFC operations with card tap/remove cycles
   • Network operations during app backgrounding
   • Handler-based operations

Testing Priority:

1. ModernPOSActivity (main POS interface)
2. PaymentRequestActivity (payment flows)
3. BalanceCheckActivity (NFC operations)
4. Activities with Handler usage

Implementation:

• Debug builds only (no impact on release)
• Provides heap dump analysis
• Shows leak traces in notifications

Priority: High - critical for identifying existing memory leaks