refactor(lobby): split LobbyScreen into per-section files

[netqo]

Summary

LobbyScreen.kt had grown to ~1100 lines, mixing the screen scaffold with every section composable (header, balance hero, games grid, quick actions, recent activity, loading skeleton). This PR moves each section into its own sibling file under feature/lobby, leaving LobbyScreen.kt (~370 lines) with just the state scaffold, the shared building blocks (Divider, NepFab) and the internal design tokens.

New files: LobbyHeader, LobbyBalanceHero, LobbyGamesSection, LobbyQuickActions, LobbyRecentActivity, LobbyLoading.

Rationale

Lobby was the outlier against Wallet, News and Assistant, which already split their sections into per-file composables. Aligning it improves navigability and keeps each file under the detekt function-count budget. While splitting, the repeated background + border boilerplate shared by the tappable cards (game card, coinflip card, quick action card) was lifted into a single Modifier.lobbyCardSurface(fill, stroke) extension.

No behavior or layout change: the public LobbyScreen API and the rendered output are identical. Verified on device.

Verification

• ./gradlew ktlintCheck detekt testDebugUnitTest -> BUILD SUCCESSFUL
• ./gradlew assembleDebug -> BUILD SUCCESSFUL
• Lobby (success / loading / error) checked on device; renders identically to before the split.

Test plan

• ktlint + detekt clean
• JVM unit tests green
• Debug APK assembles
• Lobby renders identically on device (success, loading, error states)

Checklist

• Commit messages follow Conventional Commits (see CONTRIBUTING.md).
• Branch name follows the naming convention (refactor/).
• The change is scoped: no unrelated edits sneaked into this PR.
• If user-visible behavior changed, CHANGELOG.md has been updated under [Unreleased]. (No user-visible change; pure restructuring.)
• If a new dependency was added, it is justified in the PR description. (No new dependency.)