feat(ui): add Welcome screen scaffold (#7)

[ilmoniemi]

What

Adds WelcomeScreen — a pure stateless Composable in ui/onboarding/ matching the Figma 6:32 412×892 baseline. Hero section (colored circular logo placeholder, title, subtitle), filled primary CTA I already have pyrycode invoking onPaired, outlined secondary CTA Set up pyrycode first invoking onSetup, and an open-source footer line. All colors and typography come exclusively from MaterialTheme.colorScheme.* / MaterialTheme.typography.*. Two @Preview composables (light + dark, widthDp = 412, heightDp = 892) wrapped in PyrycodeMobileTheme.

Per the architect's preferred option, the logo placeholder is a plain colored Box (no inner icon) — keeps the file dependency-free and stays out-of-scope of branding.

Issue

Closes #7

Testing

• ./gradlew assembleDebug — passes ✅
• ./gradlew test — passes ✅ (no new unit tests; AC requires Previews + assembleDebug only, per architect spec)
• ./gradlew lint — clean ✅
• ./gradlew connectedAndroidTest — not run (no device/emulator attached in this env; ticket has no instrumented-test requirement)
• grep -E 'NavController|androidx\.navigation' app/src/main/java/de/pyryco/mobile/ui/onboarding/WelcomeScreen.kt — empty (AC4)

Architecture compliance

Follows the architect's spec at docs/specs/architecture/7-welcome-screen-scaffold.md:

• Fixed signature WelcomeScreen(onPaired: () -> Unit, onSetup: () -> Unit) — no modifier parameter added (feat(ui): set up Navigation Compose with Welcome → ChannelList placeholder #8/feat(ui): wire Welcome screen CTAs to Scanner navigation + external browser #14 don't pass one).
• Top-level Surface(background) > Column(fillMaxSize, systemBarsPadding, padding 24×32, SpaceBetween, CenterHorizontally) with hero (weight(1f) + bracketing Spacer.weight(1f) for vertical centering), CTA stack, footer.
• Imports limited to material3, foundation.layout, foundation.shape.CircleShape, runtime.Composable, ui.*, unit.dp, tooling.preview.Preview, project theme — no navigation, no activity, no Intent.
• Both previews private, pass darkTheme = true/false explicitly for deterministic rendering.

[ilmoniemi]

Code Review: #7

Decision: PASS

Findings

• [NIT] WelcomeScreen.kt:52-57 — logo placeholder uses Spacer with .background() rather than Box as suggested in the spec. Functionally identical (Spacer applies its modifier chain including .background()), so renders the intended filled circle, but Box is the more idiomatic choice for an element that is meant to draw content rather than reserve empty space. Not blocking; can be left as-is or swapped at the author's discretion.

AC verification

• AC1 (signature WelcomeScreen(onPaired, onSetup) in ui/onboarding/) ✅
• AC2 (hero / primary CTA → onPaired / secondary CTA → onSetup / footer) ✅
• AC3 (colors via MaterialTheme.colorScheme.*, typography via MaterialTheme.typography.*, no Color(0x...) or hardcoded sizes/weights/families) ✅
• AC4 (no NavController / androidx.navigation.* references — confirmed via grep on the file) ✅
• AC5 (two @Preview composables, light + UI_MODE_NIGHT_YES dark, both wrapped in PyrycodeMobileTheme, both private) ✅
• AC6 (./gradlew assembleDebug — verified locally: BUILD SUCCESSFUL) ✅
• Bonus: ./gradlew lint clean.

Summary

Clean stateless Composable matching the spec layout tree. All theme tokens routed through MaterialTheme, no navigation coupling, previews are deterministic at the Figma baseline dimensions. Ready to merge; #8 and #14 can consume the exact signature without changes.