Scan the enrollment QR in-app instead of relying on the camera app

Not every phone's camera app hands a scanned aiapp:// URI off to the app
reliably, which made enrollment an annoying multi-step process. The
Settings screen now scans the QR itself via zxing-android-embedded's
ScanContract (offline, no Play Services/ML Kit download) and feeds the
decoded URI through the existing parseEnrollmentUri. The aiapp://enroll
deep link stays as a fallback for cameras that do redirect.

Verified on the emulator: Scan QR code launches the scanner, prompts for
camera permission, shows a live preview, and backing out returns to
Settings cleanly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xn8nHw1tw1R6PtiY1eEtw

app/androidApp/build.gradle.kts

@@ -126,4 +126,5 @@ dependencies {
     implementation(libs.compose.material3)
     implementation(libs.compose.ui)
     implementation(libs.androidx.activity.compose)
+    implementation(libs.zxing.embedded)
 }