feat(ui): wire Welcome onSetup CTA to external browser (#14)

[ilmoniemi]

What

Replace the TODO(#14) placeholder in MainActivity.kt's composable(Routes.Welcome) block with an Intent.ACTION_VIEW launch of https://pyryco.de/setup via LocalContext.current. The URL is held in a file-scope private const val SetupUrl next to Routes.

• WelcomeScreen's onSetup: () -> Unit parameter shape is unchanged; previews continue to pass onSetup = {} no-ops.
• Mirrors the existing onPaired wiring pattern at the same call site.
• No new dependencies; Intent, Uri, and LocalContext are already on the classpath.

Issue

Closes #14

Testing

• ./gradlew assembleDebug — BUILD SUCCESSFUL
• ./gradlew lint — BUILD SUCCESSFUL
• ./gradlew test — BUILD SUCCESSFUL
• No new tests added (per architecture spec): the change is a single SDK call wired to a callback; previews cover the screen visually; manual on-device verification covers the tap behavior.

Architecture compliance

Follows the spec at docs/specs/architecture/14-welcome-onsetup-external-browser.md exactly:

• File-scope SetupUrl constant adjacent to Routes, not inside it.
• LocalContext.current captured in the composable { ... } body and closed over by the onSetup lambda — same shape as the koinInject / rememberCoroutineScope reads in the Scanner route.
• Uri.parse(SetupUrl) over toUri() per the spec's idiom preference.
• No try/catch for ActivityNotFoundException — explicitly deferred per ticket's Out of Scope section ([[Evidence-Based Fix Selection]]).

🤖 Generated with Claude Code

[ilmoniemi]

Code Review: #14

Decision: PASS

Findings

• [NIT] MainActivity.kt:131 — SetupUrl uses PascalCase, while the project's CLAUDE.md prescribes UPPER_SNAKE_CASE for top-level const val (i.e. SETUP_URL). The architect spec explicitly chose SetupUrl and it mirrors the in-file PascalCase used inside private object Routes, so this isn't worth a re-roll, but worth surfacing for future const-naming alignment.
• [NIT] PR description references the spec path as docs/specs/architecture/14-welcome-onsetup-external-browser.md, but the file was committed as 14-welcome-setup-cta.md. Cosmetic mismatch only.

Summary

The diff is minimal, focused, and matches the architecture spec line-for-line: file-scope SetupUrl constant adjacent to Routes; LocalContext.current captured in the composable(Routes.Welcome) { ... } body and closed over by the onSetup lambda; Intent.ACTION_VIEW + Context.startActivity synchronous fire-and-forget; no try/catch (deferred per ticket's Out of Scope, [[Evidence-Based Fix Selection]] principle); WelcomeScreen signature and previews untouched. Uri.parse over toUri() per spec preference.

codegraph_callers WelcomeScreen confirms the only call sites are MainActivity + the in-file previews — no missed signature-change ripple. All ACs are satisfied; builds, lint, and tests pass per the PR description.