feat: add demo mode for testing and review purposes

- Activates by tapping "None (disable)" 7 times.
- Displays a confirmation message when `Demo Mode` is enabled.
- Simulates a connection to a Meshtastic device and allows the app to function without requiring real hardware.

app/src/main/java/com/geeksville/mesh/ui/SettingsFragment.kt

@@ -290,6 +290,9 @@ class SettingsFragment : ScreenFragment("Settings"), Logging {
             .show()
     }
 
+    private var tapCount = 0
+    private var lastTapTime: Long = 0
+
     private fun addDeviceButton(device: BTScanModel.DeviceListEntry, enabled: Boolean) {
         val b = RadioButton(requireActivity())
         b.text = device.name
@@ -299,6 +302,19 @@ class SettingsFragment : ScreenFragment("Settings"), Logging {
         binding.deviceRadioGroup.addView(b)
 
         b.setOnClickListener {
+            if (device.fullAddress == "n") {
+                val currentTapTime = System.currentTimeMillis()
+                if (currentTapTime - lastTapTime > TAP_THRESHOLD) {
+                    tapCount = 0
+                }
+                lastTapTime = currentTapTime
+                tapCount++
+
+                if (tapCount >= TAP_TRIGGER) {
+                    model.showSnackbar("Demo Mode enabled")
+                    scanModel.showMockInterface()
+                }
+            }
             if (!device.bonded) // If user just clicked on us, try to bond
                 binding.scanStatusText.setText(R.string.starting_pairing)
             b.isChecked = scanModel.onSelected(device)
@@ -354,7 +370,7 @@ class SettingsFragment : ScreenFragment("Settings"), Logging {
         // If we are running on an emulator, always leave this message showing so we can test the worst case layout
         val curRadio = scanModel.selectedAddress
 
-        if (curRadio != null && !scanModel.isMockInterfaceAddressValid) {
+        if (curRadio != null && curRadio != "m") {
             binding.warningNotPaired.visibility = View.GONE
         } else if (bluetoothViewModel.enabled.value == true) {
             binding.warningNotPaired.visibility = View.VISIBLE
@@ -449,6 +465,9 @@ class SettingsFragment : ScreenFragment("Settings"), Logging {
 
     companion object {
         const val SCAN_PERIOD: Long = 10000 // Stops scanning after 10 seconds
+        private const val TAP_TRIGGER: Int = 7
+        private const val TAP_THRESHOLD: Long = 500 // max 500 ms between taps
+
     }
 
     private fun Editable.isIPAddress(): Boolean {