feat(ui): add Stub Scanner screen + route (#12)

[ilmoniemi]

What

• New ScannerScreen composable in ui/onboarding/ — full-screen tap-anywhere surface with a viewport box + scan-line indicator + "Tap to pair" affordance. No real camera (CameraX + ML Kit are Phase 4).
• New Routes.Scanner = "scanner" constant and composable(Routes.Scanner) { ... } block in MainActivity.kt. On tap: appPreferences.setPairedServerExists(true) then navigate to channel_list with popUpTo(Routes.Scanner) { inclusive = true } + launchSingleTop = true.
• Re-wired Welcome's onPaired to navigate to Routes.Scanner instead of straight to Routes.ChannelList.
• Light + dark @Preview functions on ScannerScreen.

Issue

Closes #12

Testing

• ./gradlew assembleDebug — passes
• ./gradlew test — passes (no new unit tests; matches the precedent set by Welcome / Settings / ConversationThread placeholders — see spec "Testing strategy")
• ./gradlew lint — passes
• ./gradlew connectedAndroidTest — not run (no device/emulator attached in this environment)

Architecture compliance

• Matches WelcomeScreen.kt shape exactly: outer Surface(color = background, fillMaxSize), inner Column with systemBarsPadding(), two widthDp = 412, heightDp = 892 previews using PyrycodeMobileTheme.
• AppPreferences resolved via koinInject<AppPreferences>() (org.koin.compose) inside the composable(...) block — no ViewModel for a stub that has no observable state and will be replaced wholesale in Phase 4.
• rememberCoroutineScope() for the suspend write; sequential write-then-navigate so pairedServerExists is flipped before the next destination composes (matters once feat(ui): conditional NavHost start destination based on paired-server-exists #13 lands).
• pointerInput(Unit) { detectTapGestures { onTap() } } for tap-anywhere with no ripple — Modifier.clickable would fight the camera-viewport metaphor with a Material ripple over the whole screen.
• popUpTo(Routes.Scanner) { inclusive = true } satisfies AC3 (back-press from ChannelList does not return to Scanner). Welcome is intentionally left in the back stack — feat(ui): conditional NavHost start destination based on paired-server-exists #13 owns the conditional start destination.

🤖 Generated with Claude Code

[ilmoniemi]

Code Review: #12

Decision: PASS

Findings

None blocking. The implementation matches the architect's spec verbatim — route constant, Welcome re-wire, composable(Routes.Scanner) block (with koinInject<AppPreferences>() + rememberCoroutineScope() + sequential setPairedServerExists(true) → navigate(... popUpTo(Routes.Scanner) { inclusive = true }, launchSingleTop = true)), and ScannerScreen.kt composition shape (Surface + pointerInput on the outer Surface, viewport Box with aspectRatio(1f) + surfaceVariant, scan-line, "Tap to pair" affordance, light + dark previews).

Spot checks:

• Material 3 tokens — every color and typography reference goes through MaterialTheme.colorScheme.* / MaterialTheme.typography.*. No hardcoded Color(0xFF...) or TextStyle() defaults. ✓
• Recomposition correctness — pointerInput(Unit) keyed on Unit is intentional and documented in the spec's open-questions section; onTap is bound to a stable navigation closure. ✓
• Coroutines — rememberCoroutineScope() (not GlobalScope, not a fresh CoroutineScope), sequential await before navigation so the preference flip is durable before feat(ui): conditional NavHost start destination based on paired-server-exists #13's collector reads it. ✓
• Back stack — popUpTo(Routes.Scanner) { inclusive = true } + launchSingleTop = true satisfies AC3 ("scanner removed from the back stack") and guards against double-tap stacking. ✓
• Accessibility — whole-screen tap target with a visible "Tap to pair" label; no icon-only elements that would need contentDescription. ✓
• Edge-to-edge — pointerInput lives on the Surface (catches taps in the inset region), systemBarsPadding() lives on the inner Column (content avoids the status-bar overlap). Matches WelcomeScreen.kt. ✓
• Previews — both render in isolation (no Koin/NavController/coroutine dependencies in the composable body). ✓
• Tests — none required per the spec's testing-strategy section, which aligns with the WelcomeScreen / Settings / ConversationThread placeholder precedent in this Phase-0 codebase. ✓

Summary

Clean, minimal, spec-faithful stub. Phase 4 will replace the whole composable(Routes.Scanner) { ... } block plus ScannerScreen.kt wholesale (CameraX + ML Kit); the Routes.Scanner constant is the only piece that survives. No premature abstractions anticipating the rewrite — appropriate. Ready to merge.