Add comprehensive documentation (KDoc) for public APIs

[callebtc]

Description:
Many public APIs lack proper KDoc documentation, making the codebase harder to understand and maintain.

Missing Documentation Areas:

• Public class constructors
• Public method parameters and return values
• Manager classes and their lifecycle
• Extension functions
• Data classes and their properties

Impact:

1. Poor developer experience for team members
2. Unclear API contracts
3. Harder onboarding for new developers

Fix Instructions:
Add comprehensive KDoc for all public APIs:

/**
 * Manages Cashu wallet operations and database connections.
 * 
 * This singleton class handles:
 * - MultiMintWallet initialization
 * - Database lifecycle management  
 * - Mint configuration changes
 * 
 * @property appContext The application context for database access
 * @see MultiMintWallet
 */
object CashuWalletManager {

    /**
     * Initializes the wallet manager with the given context.
     * Safe to call multiple times.
     * 
     * @param context The application context
     * @throws IllegalStateException if initialization fails
     */
    fun init(context: Context) { ... }
}

Priority Areas:

1. CashuWalletManager - wallet operations
2. PosUiCoordinator - UI coordination
3. Payment-related classes
4. Manager classes

Tools: Use Android Studio's "Generate Documentation" feature