Implement Memory Clearing for Sensitive Data

## Priority: Medium

Explicitly zero out memory containing sensitive data to prevent memory dump attacks.

## Current State
- Swift Data structures deallocated normally
- Sensitive data may remain in memory after use

## Implementation

```swift
extension Data {
mutating func securelyZero() {
withUnsafeMutableBytes { bytes in
memset(bytes.baseAddress!, 0, bytes.count)
}
}
}

// Usage:
var sensitiveData = Data(...)
defer { sensitiveData.securelyZero() }
```

## Apply To
- Decrypted message content
- Temporary encryption keys
- User passwords (if cached)
- JWT tokens after processing

## Benefits
- Prevents memory dump attacks
- Reduces attack surface
- Defense in depth

## References
Part of security audit findings

The issue proposes a Swift Data extension and identifies decrypted messages, temporary encryption keys, cached passwords, and JWT tokens as targets. Start by locating those data flows in the app; no file or test is named in the issue. Done means sensitive values are explicitly cleared after use across the affected paths.

Written by the indexing model from the issue text.