feat(ui): set up Navigation Compose with Welcome → ChannelList placeholder (#8)

[ilmoniemi]

What

Replaces the AGP-template Greeting("Android") in MainActivity with a NavHost rooted at WelcomeScreen and adds a channel_list placeholder destination.

• gradle/libs.versions.toml — adds navigationCompose = "2.9.5" and the androidx-navigation-compose library alias (Navigation Compose is not in the Compose BOM).
• app/build.gradle.kts — adds implementation(libs.androidx.navigation.compose) (alphabetically placed; no inline version literal).
• MainActivity.kt — deletes the template Greeting and GreetingPreview, introduces a private PyryNavHost that owns rememberNavController(), and a private object Routes with Welcome / ChannelList string constants. onPaired navigates to channel_list; onSetup is a no-op with a TODO(#14) comment.

Issue

Closes #8

Testing

• ./gradlew assembleDebug — passes (the only AC-binding gate).
• ./gradlew test — passes.
• ./gradlew lint — passes (clean).
• ./gradlew connectedAndroidTest — not run, no emulator/device attached in this environment.

No navigation tests added per the spec's "Testing strategy" section (instrumentation tests deferred; spec explicitly says not to add androidx-navigation-testing for this ticket).

Architecture compliance

Follows docs/specs/architecture/8-navigation-compose-welcome-channellist-placeholder.md exactly:

• PyryNavHost extracted as a private Composable inside MainActivity.kt, keeping setContent at a single level of abstraction (theme → scaffold → nav host).
• Route strings live in a single private object Routes; no sealed class Route / type-safe navigation refactor.
• onPaired uses plain navController.navigate(Routes.ChannelList) — no popUpTo / launchSingleTop. Back from channel_list returns to welcome, as expected for this scaffolding ticket.
• channel_list route renders bare Text("Channel list placeholder") — no Surface, no Scaffold, no padding, as specified.
• enableEdgeToEdge() and the outer Scaffold(modifier = Modifier.fillMaxSize()) preserved; WelcomeScreen is untouched.

🤖 Generated with Claude Code

[ilmoniemi]

Code Review: #8

Decision: PASS

Findings

None.

Summary

Implementation matches the architect's spec exactly. All six acceptance criteria are satisfied:

• navigationCompose = "2.9.5" added to [versions], androidx-navigation-compose alias added to [libraries], consumed via libs.androidx.navigation.compose in app/build.gradle.kts — no inline version literal (AC1).
• Greeting and GreetingPreview deleted; androidx.compose.ui.tooling.preview.Preview import removed cleanly (AC2).
• NavHost with welcome (start) and channel_list destinations, extracted into a private PyryNavHost Composable that owns rememberNavController() — stateless host pattern per the ticket's Technical Notes (AC3).
• WelcomeScreen wired with onPaired = { navController.navigate(Routes.ChannelList) } and an inert onSetup no-op carrying the TODO(#14) comment (AC4).
• channel_list route body is the bare Text("Channel list placeholder") — no extra chrome, matching the spec's instruction that the outer Scaffold provides padding (AC5).
• ./gradlew assembleDebug passes; ./gradlew lint and ./gradlew test are also clean (AC6).

Route strings centralised in a private object Routes, PyryNavHost is private, no back-stack tuning, no deep links, no test deps added — all in line with the spec's "do not implement" list. Trailing newline restored on MainActivity.kt (the old file was missing one).