[WIP] [SUB-TASK] Implement Biometric Authentication

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Sub-task of #84: Security Authentication System

Description

Implement proper biometric authentication using LocalAuthentication framework for Face ID and Touch ID support.

Current State

• LocalAuthentication framework imported but not used
• Mock implementation in place
• No biometric enrollment check
• Missing fallback mechanisms

Implementation Requirements

• Create BiometricAuthService with LocalAuthentication
• Check device biometric capabilities
• Implement Face ID/Touch ID flows
• Add passcode fallback
• Handle biometric changes
• Store biometric-protected items in Keychain

Technical Implementation

import LocalAuthentication

@MainActor
class BiometricAuthService: ObservableObject {
    private let context = LAContext()
    @Published var isAvailable = false
    @Published var biometryType: LABiometryType = .none
    
    func checkBiometricAvailability() {
        var error: NSError?
        isAvailable = context.canEvaluatePolicy(
            .deviceOwnerAuthenticationWithBiometrics,
            error: &error
        )
        biometryType = context.biometryType
    }
    
    func authenticate(reason: String) async -> Result<Bool, AuthError> {
        do {
            let success = try await context.evaluatePolicy(
                .deviceOwnerAuthenticationWithBiometrics,
                localizedReason: reason
            )
            return .success(success)
        } catch {
            return .failure(mapLAError(error))
        }
    }
}

UI Requirements

• Biometric prompt with clear reason
• Settings toggle for biometric auth
• Enrollment guidance
• Error state handling

Error Scenarios

• Biometrics not available
• Biometrics not enrolled
• Authentication failed
• Too many attempts
• Biometrics changed

Acceptance Criteria

• Face ID works on supported devices
• Touch ID works on supported devices
• Passcode fallback available
• Settings toggle functional
• All error states handled

Estimated Effort: 3-4 days

Parent Issue: #84

Fixes #107.

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