Add FT-991A Remote Control App for macOS

Complete Phase 1 implementation of the Yaesu FT-991A remote control
application with CAT protocol support over USB serial (CP210x).

Features implemented:
- SerialPortManager with auto-detection of CP210x ports
- Full CAT protocol parser and command builder
- RadioState model with all transceiver parameters
- Modern SwiftUI interface with frequency/mode/level controls
- Skeuomorphic front panel view (switchable)
- Debug panel with CAT command console
- QSO log panel with CSV export/import
- Audio routing panel with BlackHole integration
- Settings with connection, UI, keyboard configuration
- Menu bar extra for background operation
- German/English localization
- Logging system for debugging

Supports: Frequency control, VFO A/B, all modes (LSB/USB/CW/FM/AM/
DATA/RTTY/C4FM), level controls, NB/NR/DNF/ATU/Split functions,
S-meter/Power/SWR metering, PTT control via Shift key.

Target: macOS 15.0+ (Sequoia/Tahoe)

FT991A-Remote/FT991A-Remote/ViewModels/RadioViewModel.swift

@@ -0,0 +1,306 @@
+//
+//  RadioViewModel.swift
+//  FT991A-Remote
+//
+//  Main ViewModel for radio control
+//
+
+import Foundation
+import Combine
+import SwiftUI
+
+// MARK: - Radio ViewModel
+
+@MainActor
+class RadioViewModel: ObservableObject {
+
+    // MARK: - Published Properties
+
+    // Connection
+    @Published var isConnected = false
+    @Published var connectionState: ConnectionState = .disconnected
+    @Published var availablePorts: [SerialPort] = []
+    @Published var selectedPort: String = ""
+    @Published var baudRate: Int = 38400
+
+    // Radio State (mirrored for convenience)
+    @Published var vfoAFrequency: Int = 14_250_000
+    @Published var vfoBFrequency: Int = 14_255_000
+    @Published var activeVFO: VFO = .a
+    @Published var mode: OperatingMode = .usb
+    @Published var frequencyStep: FrequencyStep = .khz1
+
+    // Levels
+    @Published var afGain: Int = 128
+    @Published var rfGain: Int = 255
+    @Published var squelch: Int = 0
+    @Published var micGain: Int = 50
+    @Published var power: Int = 100
+
+    // Functions
+    @Published var noiseBlanker = false
+    @Published var noiseReduction = false
+    @Published var dnf = false
+    @Published var contour = false
+    @Published var atu = false
+    @Published var split = false
+    @Published var ipo = false
+
+    // Metering
+    @Published var sMeter: Int = 0
+    @Published var powerMeter: Int = 0
+    @Published var swrMeter: Int = 0
+    @Published var isTransmitting = false
+
+    // Statistics
+    @Published var bytesSent: UInt64 = 0
+    @Published var bytesReceived: UInt64 = 0
+
+    // Debug
+    @Published var commandHistory: [CommandLogEntry] = []
+
+    // MARK: - Services
+
+    private let serialManager = SerialPortManager()
+    private let catProtocol: CATProtocol
+    private var cancellables = Set<AnyCancellable>()
+
+    // MARK: - Computed Properties
+
+    var activeFrequency: Int {
+        activeVFO == .a ? vfoAFrequency : vfoBFrequency
+    }
+
+    var frequencyDisplay: String {
+        formatFrequency(activeFrequency)
+    }
+
+    var sMeterDisplay: String {
+        let normalized = Double(sMeter) / 255.0
+        if normalized <= 0.6 {
+            let sUnit = Int(normalized / 0.6 * 9.0)
+            return "S\(sUnit)"
+        } else {
+            let db = Int((normalized - 0.6) / 0.4 * 60.0)
+            return "S9+\(db)"
+        }
+    }
+
+    var currentBand: Band? {
+        Band.from(frequency: activeFrequency)
+    }
+
+    // MARK: - Initialization
+
+    init() {
+        catProtocol = CATProtocol(serialManager: serialManager)
+        setupBindings()
+        refreshPorts()
+    }
+
+    private func setupBindings() {
+        // Serial Manager bindings
+        serialManager.$connectionState
+            .receive(on: DispatchQueue.main)
+            .sink { [weak self] state in
+                self?.connectionState = state
+                self?.isConnected = state.isConnected
+            }
+            .store(in: &cancellables)
+
+        serialManager.$availablePorts
+            .receive(on: DispatchQueue.main)
+            .assign(to: &$availablePorts)
+
+        serialManager.$selectedPortPath
+            .receive(on: DispatchQueue.main)
+            .assign(to: &$selectedPort)
+
+        serialManager.$bytesSent
+            .receive(on: DispatchQueue.main)
+            .assign(to: &$bytesSent)
+
+        serialManager.$bytesReceived
+            .receive(on: DispatchQueue.main)
+            .assign(to: &$bytesReceived)
+
+        // CAT Protocol bindings
+        catProtocol.$radioState
+            .receive(on: DispatchQueue.main)
+            .sink { [weak self] state in
+                self?.updateFromRadioState(state)
+            }
+            .store(in: &cancellables)
+
+        catProtocol.$commandHistory
+            .receive(on: DispatchQueue.main)
+            .assign(to: &$commandHistory)
+    }
+
+    private func updateFromRadioState(_ state: RadioState) {
+        vfoAFrequency = state.vfoAFrequency
+        vfoBFrequency = state.vfoBFrequency
+        activeVFO = state.activeVFO
+        mode = state.mode
+        afGain = state.afGain
+        rfGain = state.rfGain
+        squelch = state.squelch
+        micGain = state.micGain
+        power = state.power
+        noiseBlanker = state.noiseBlanker
+        noiseReduction = state.noiseReduction
+        dnf = state.dnf
+        contour = state.contour
+        atu = state.atu
+        split = state.split
+        ipo = state.ipo
+        sMeter = state.sMeter
+        powerMeter = state.powerMeter
+        swrMeter = state.swrMeter
+        isTransmitting = state.isTransmitting
+    }
+
+    // MARK: - Connection
+
+    func refreshPorts() {
+        serialManager.refreshPorts()
+    }
+
+    func connect() {
+        serialManager.selectedPortPath = selectedPort
+        serialManager.baudRate = baudRate
+        serialManager.connect()
+    }
+
+    func disconnect() {
+        serialManager.disconnect()
+    }
+
+    func toggleConnection() {
+        if isConnected {
+            disconnect()
+        } else {
+            connect()
+        }
+    }
+
+    func selectPort(_ path: String) {
+        selectedPort = path
+        serialManager.selectedPortPath = path
+    }
+
+    // MARK: - Frequency Control
+
+    func setFrequency(_ frequency: Int) {
+        catProtocol.setFrequency(frequency, vfo: activeVFO)
+    }
+
+    func incrementFrequency() {
+        catProtocol.changeFrequency(by: frequencyStep.rawValue)
+    }
+
+    func decrementFrequency() {
+        catProtocol.changeFrequency(by: -frequencyStep.rawValue)
+    }
+
+    func selectBand(_ band: Band) {
+        catProtocol.selectBand(band)
+    }
+
+    // MARK: - VFO Control
+
+    func selectVFO(_ vfo: VFO) {
+        catProtocol.selectVFO(vfo)
+    }
+
+    func swapVFO() {
+        catProtocol.swapVFO()
+    }
+
+    func equalizeVFO() {
+        catProtocol.equalizeVFO()
+    }
+
+    // MARK: - Mode Control
+
+    func setMode(_ mode: OperatingMode) {
+        catProtocol.setMode(mode)
+    }
+
+    // MARK: - Level Control
+
+    func setAFGain(_ value: Int) {
+        catProtocol.setAFGain(value)
+    }
+
+    func setRFGain(_ value: Int) {
+        catProtocol.setRFGain(value)
+    }
+
+    func setSquelch(_ value: Int) {
+        catProtocol.setSquelch(value)
+    }
+
+    func setMICGain(_ value: Int) {
+        catProtocol.setMICGain(value)
+    }
+
+    func setPower(_ value: Int) {
+        catProtocol.setPower(value)
+    }
+
+    // MARK: - Function Control
+
+    func toggleNB() {
+        catProtocol.toggleNB()
+    }
+
+    func toggleNR() {
+        catProtocol.toggleNR()
+    }
+
+    func toggleDNF() {
+        catProtocol.toggleDNF()
+    }
+
+    func toggleSplit() {
+        catProtocol.toggleSplit()
+    }
+
+    func startATUTune() {
+        catProtocol.startATUTune()
+    }
+
+    // MARK: - PTT Control
+
+    func startTransmit(dataMode: Bool = false) {
+        catProtocol.startTransmit(dataMode: dataMode)
+    }
+
+    func stopTransmit() {
+        catProtocol.stopTransmit()
+    }
+
+    func toggleTransmit(dataMode: Bool = false) {
+        catProtocol.toggleTransmit(dataMode: dataMode)
+    }
+
+    // MARK: - Debug
+
+    func sendRawCommand(_ command: String) {
+        catProtocol.sendRaw(command)
+    }
+
+    func clearCommandHistory() {
+        catProtocol.clearCommandHistory()
+    }
+
+    // MARK: - Helpers
+
+    func formatFrequency(_ freq: Int) -> String {
+        let mhz = freq / 1_000_000
+        let khz = (freq % 1_000_000) / 1_000
+        let hz = freq % 1_000
+        return String(format: "%d.%03d.%03d", mhz, khz, hz)
+    }
+}