🟡 MEDIUM: Standardize on async/await throughout codebase

[arkavo-com]

Problem

The codebase mixes completion handlers and async/await patterns inconsistently.

Examples

// AuthenticationManager.swift - closure-based
func registrationOptions(accountName: String, completion: @escaping (Data?, Data?) -> Void)

// ArkavoApp.swift - async/await
private func saveProfile(profile: Profile) async -> Bool

Impact

• Callback hell in some code paths
• Inconsistent error handling patterns
• Difficulty composing async operations
• Mental overhead switching between patterns

Solution

1. Convert all completion handlers to async/await
2. Use withCheckedContinuation for bridging legacy APIs
3. Standardize error handling with throws
4. Update all call sites

Files to Update

• AuthenticationManager.swift - Multiple completion handlers
• ArkavoClient callbacks
• Network layer methods

Acceptance Criteria

• All public APIs use async/await
• No completion handlers in new code
• Consistent throws for error handling
• Legacy bridging documented where needed