Add NFC tag support with unified device selection

[Copilot]

This PR implements comprehensive NFC tag support for TwoKey, extending the existing CCID-only functionality to work with NFC FIDO2 authenticators. The implementation provides an elegant device selection mechanism when multiple authenticator types are available.

Overview

The changes introduce a unified FIDO API architecture that seamlessly handles both CCID readers and NFC tags through a single interface. Users can now authenticate using NFC-enabled FIDO2 keys on mobile devices while maintaining full backwards compatibility with existing CCID workflows.

Key Features

🔧 Unified Device Management

• UnifiedFidoApi: New abstraction layer that automatically detects and manages both CCID and NFC devices
• Automatic detection: Scans for available authenticators on connection
• Smart defaults: Prefers CCID over NFC for stability when both are available
• Device switching: Users can manually select their preferred authenticator type

📱 NFC Implementation

• NfcFidoApi: Complete NFC implementation using flutter_nfc_kit
• Transient connection handling: Automatic reconnection for lost NFC connections
• Platform-aware: Only enables NFC on mobile devices where it's supported
• Enhanced error handling: Clear feedback for NFC-specific issues like "hold key near device"

🎨 Enhanced User Interface

• Device selection dialog: Clean UI for choosing between multiple authenticators
• Current device display: Shows active authenticator in device info section
• Refresh capabilities: Manual device list refresh and connection reset
• Visual indicators: USB icon for CCID, NFC icon for wireless devices

Technical Implementation

The architecture follows the existing FidoApi interface pattern, ensuring zero breaking changes:

// Before: Direct CCID usage
AuthenticatorService(CcidFidoApi())

// After: Unified device support  
AuthenticatorService(UnifiedFidoApi())

NFC Lifecycle Management

The implementation handles NFC's transient nature through:

• Automatic reconnection: Transparent recovery from lost connections during transactions
• Enhanced transceive: transceiveWithReconnect() method for robust communication
• Connection polling: Regular availability checks for better reliability

Device Selection Flow

1. App scans for available FIDO2 devices (CCID readers + NFC availability)
2. Single device → connects automatically
3. Multiple devices → presents selection dialog
4. User choice is remembered for the session
5. Manual refresh available via UI buttons

Files Changed

New APIs:

• lib/api/nfc_fido_api.dart - NFC FIDO implementation
• lib/api/unified_fido_api.dart - Unified device selector

Enhanced Components:

• lib/viewmodels/keys.dart - Device selection state management
• lib/views/keys/keys.dart - Device selection dialog
• lib/service/authenticator.dart - Device enumeration methods
• lib/views/keys/widgets/device_info_section.dart - Current device display

Backwards Compatibility

✅ Zero breaking changes - existing CCID workflows continue unchanged
✅ Graceful degradation - falls back to CCID-only on non-mobile platforms
✅ Error handling - clear messages when NFC is unavailable

Fixes #2

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support NFC tags</issue_title>
<issue_description>We should support NFC tags in addition to CCID with flutter_nfc_kit. Some suggestions:

• extend the FidoApi class with nfc_fido_api.
• design an elegant way of choosing tags when (potentially multiple) CCID readers and NFC are available

Please note that NFC communication might be transient -- so the APIs and lifecycle management might need to be redesigned, i.e. you need to check & poll tags every time when the user triggers communication.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #2

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[Harry-Chen]

@copilot The tests you write do not look very useful, please remove them from the PR.

[Copilot]

@copilot The tests you write do not look very useful, please remove them from the PR.

Removed the test files as requested. The commit b8eb0e2 removes both test/api/unified_fido_api_test.dart and test/integration_test.dart that I had added previously.