feat(aprs): link out to request a passcode instead of computing one

The settings card had a "Compute passcode" button that derived the value from the
callsign and filled the field in. It was added by request, so it stayed while the
previous commit removed the same derivation from the connection path - which left
the app contradicting itself: the background no longer invented a passcode, but the
UI still offered to.

APRS-IS treats the passcode as a licence check and states that supplying it to a
user is the software author's responsibility. APRSdroid carries the identical
algorithm in the same source file and deliberately does not use it for this, opting
to validate what the operator typed and link out to request one. Filling the field
in claims a check that nobody performed.

The button now opens the passcode request page. AprsPacket.passcode stays in
core:domain because validating an entry means recomputing the expected value, and
its import is dropped from the card, which no longer needs it.

feature/settings/src/main/java/com/rtbishop/look4sat/feature/settings/AprsCard.kt

@@ -2,6 +2,7 @@ package com.rtbishop.look4sat.feature.settings
 
 import android.content.Intent
 import android.content.pm.PackageManager
+import android.net.Uri
 import android.os.Build
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
@@ -33,12 +34,19 @@ import androidx.compose.ui.text.input.KeyboardType
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
 import com.rtbishop.look4sat.core.data.aprs.AprsConfig
-import com.rtbishop.look4sat.core.domain.aprs.AprsPacket
 import com.rtbishop.look4sat.core.data.aprs.AprsStore
 import com.rtbishop.look4sat.core.presentation.CardButton
 import com.rtbishop.look4sat.core.presentation.R
 
 /** APRS card: sits between the satellite-data and data-output containers; settings live in a gear dialog to save space */
+/**
+ * Where an operator obtains an APRS-IS passcode.
+ *
+ * The app links here rather than deriving one: the passcode is a licence check, and APRS-IS
+ * states that supplying it is the software author's responsibility.
+ */
+private const val PASSCODE_REQUEST_URL = "https://apps.magicbug.co.uk/passcode/"
+
 @Composable
 fun AprsCard() {
     val context = LocalContext.current
@@ -187,6 +195,7 @@ private fun AprsSettingsDialog(
     var symbolTable by remember { mutableStateOf(config.symbolTable) }
     var symbolCode by remember { mutableStateOf(config.symbolCode) }
     val textStyle = LocalTextStyle.current.copy(fontSize = 14.sp)
+    val context = LocalContext.current
 
     AlertDialog(
         onDismissRequest = onDismiss,
@@ -215,16 +224,19 @@ private fun AprsSettingsDialog(
                     singleLine = true,
                     textStyle = textStyle
                 )
-                // Passcode compute button (user request: let the user see the computed result)
+                // Links out instead of computing it. APRS-IS treats the passcode as a licence
+                // check and says supplying it to a user is the software author's job; APRSdroid
+                // carries the same algorithm and deliberately does not use it here for that
+                // reason. Filling the field in claims a check that nobody performed.
                 TextButton(
                     onClick = {
-                        val call = callsign.trim().uppercase()
-                        if (call.isNotBlank()) {
-                            passcode = AprsPacket.passcode(call).toString()
+                        runCatching {
+                            context.startActivity(
+                                Intent(Intent.ACTION_VIEW, Uri.parse(PASSCODE_REQUEST_URL))
+                            )
                         }
-                    },
-                    enabled = callsign.trim().isNotBlank()
-                ) { Text(stringResource(id = R.string.prefs_aprs_calc_passcode)) }
+                    }
+                ) { Text(stringResource(id = R.string.prefs_aprs_request_passcode)) }
                 Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
                     OutlinedTextField(
                         value = ssid,