Scan the enrollment QR in either polarity

ai-server prints the QR as block characters, which take the terminal's
foreground colour. On a dark-themed terminal that comes out as a
photographic negative, and ZXing looks only for a dark code on a light
ground, so the in-app scanner silently never matched it -- while the
phone's own camera app, which tries both, did.

Fixing it in the scanner rather than by having the server force its
colours means however the code gets displayed stops mattering: a light
terminal, a dark one, a screenshot someone inverted, a printout.

MIXED_SCAN alternates normal and inverted frames, so the cost is half the
frame rate at each polarity. Checked against the library's own
MixedDecoder off-device: the plain decoder reads the normal image and
returns nothing for the inverted one, and the mixed decoder reads both
within two frames.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QQz6R4kBQcWSHBNZMgBnjL

app/androidApp/src/main/kotlin/com/example/aiapp/SettingsScreen.kt

@@ -24,6 +24,7 @@ import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.unit.dp
 import androidx.activity.compose.rememberLauncherForActivityResult
+import com.google.zxing.client.android.Intents
 import com.journeyapps.barcodescanner.ScanContract
 import com.journeyapps.barcodescanner.ScanIntentResult
 import com.journeyapps.barcodescanner.ScanOptions
@@ -82,7 +83,22 @@ fun SettingsScreen(
         Spacer(Modifier.height(16.dp))
 
         OutlinedButton(
-            onClick = { scanLauncher.launch(ScanOptions().setDesiredBarcodeFormats(ScanOptions.QR_CODE)) },
+            onClick = {
+                scanLauncher.launch(
+                    ScanOptions()
+                        .setDesiredBarcodeFormats(ScanOptions.QR_CODE)
+                        // Decode both polarities: ZXing otherwise looks only for a
+                        // dark code on a light ground, and ai-server's QR is block
+                        // characters in the terminal's foreground colour -- so on a
+                        // dark-themed terminal it comes out as a photographic
+                        // negative that the scanner silently never matches. Which
+                        // way round it renders is the terminal's business, not
+                        // something this app should depend on. MIXED_SCAN alternates
+                        // normal and inverted frames, so it costs half the frame
+                        // rate at each polarity and nothing else.
+                        .addExtra(Intents.Scan.SCAN_TYPE, Intents.Scan.MIXED_SCAN)
+                )
+            },
             modifier = Modifier.fillMaxWidth(),
         ) { Text("Scan QR code") }
         Spacer(Modifier.height(16.dp))