feat(ui): add Settings screen placeholder route to NavHost

[ilmoniemi]

User Story

As a developer wiring up Pyrycode Mobile's UI scaffolding, I want an empty-but-routable settings destination registered in the NavHost so that the Channel List TopAppBar's gear icon (to be wired in a later ticket) has a real route to navigate to, and so that Phase 3 can build the actual Settings sections inside this scaffold without re-routing.

Context

The NavHost from #8 currently registers two routes: welcome and channel_list. Settings is a Phase 3 surface (Connection / Appearance / Notifications / Memory / About), but the entrypoint affordance — a gear icon in the Channel List TopAppBar — will land before Phase 3. To keep that wiring ticket trivial, we need the destination to exist as a placeholder now.

The placeholder is intentionally empty: a single Text("Settings placeholder") plus a back-navigation affordance. Wiring the gear icon itself is out of scope — that lives with whatever ticket introduces the Channel List TopAppBar.

Acceptance Criteria

• A settings route is registered in MainActivity's NavHost, alongside the existing welcome and channel_list routes.
• The route's content is a stateless Composable that renders Text("Settings placeholder") and a back-navigation affordance (e.g. an IconButton with Icons.AutoMirrored.Filled.ArrowBack, or equivalent Material 3 back-button pattern).
• Tapping the back-navigation affordance calls navController.popBackStack() (or equivalent) so the previous destination is restored.
• Navigating to settings from any source route renders the placeholder; navigating back returns to the source route.
• ./gradlew assembleDebug passes.

Technical Notes

• Add the route constant alongside the existing Routes object in MainActivity.kt (const val Settings = "settings").
• Keep the placeholder Composable stateless — it receives an onBack: () -> Unit and the route block in NavHost supplies { navController.popBackStack() }.
• No ViewModel, no DI wiring, no theme work beyond what the existing scaffold provides.

Out of Scope

• Real Settings sections — Phase 3 builds Connection / Appearance / Notifications / Memory / About.
• Wiring the Channel List TopAppBar gear icon to navigate here — separate ticket once the TopAppBar exists.
• Deep-link support or argument passing for the settings route.

Depends On

• feat(ui): set up Navigation Compose with Welcome → ChannelList placeholder #8 (NavHost) — closed/merged.

Size Estimate

XS — single file touched (MainActivity.kt), ~15–25 LOC of production code, no new packages or dependencies.

[ilmoniemi]

🤖 Product Owner — creates structured issues

po agent has completed work on this ticket.

Agent output (click to expand)

Refined #16 into the standard PO format (User Story / Context / Acceptance Criteria / Technical Notes / Out of Scope / Depends On / Size Estimate), kept `size:xs` (single file, ~15–25 LOC), and preserved the original scope: empty `settings` route + back affordance, gear-icon wiring explicitly out of scope. Not security-sensitive (placeholder UI, no input). Dispatcher will auto-add `ready:po`.

Ready for human review. Move to the next column when approved.

[ilmoniemi]

🤖 System Architect — defines Go interfaces, data flows, concurrency patterns

architect agent has completed work on this ticket.

Agent output (click to expand)

Spec committed at `024ffd0` on `feature/16`.

**Summary:** Sized XS (1 file, ~20 LOC, no fan-out, no overlap, no security label). Spec at `docs/specs/architecture/16-settings-placeholder-route.md` follows #15's pattern but factors out a `private SettingsPlaceholder(onBack)` composable since the body has two elements + a callback. **Key non-obvious call:** uses `TextButton("Back")` instead of `IconButton` + `Icons.AutoMirrored.Filled.ArrowBack` because `material-icons-core` is not on the classpath (verified via `gradlew app:dependencies` and AAR inspection). AC2 explicitly allows "equivalent Material 3 back-button pattern", and adding a new dep for a throwaway placeholder belongs to the future TopAppBar ticket where the cost amortizes. Next: PO moves to In Development; developer picks it up.

Ready for human review. Move to the next column when approved.