fix(splash): stop launch logo bleeding through transparent screens

[bmc08gt]

Problem

Users see the Flipcash "F" launch logo (white rounded-pill mark, no wordmark) stuck on screen over fully-interactive content — My Account settings, the Wallet tab, and chat threads — across the entire session. The logo appears centered on those screens but is absent on the You tab and Chat tab home.

Root cause

android:windowBackground in Theme.Code is set to @drawable/splash (a layer-list: splash_background color + ic_launcher_foreground centered). This drawable is never cleared after MainActivity.setContent {}. Once Compose renders its first frame the window background sits in the DecorView behind the Compose surface for the Activity's entire lifetime.

The outermost Compose root Box in App.kt had no background modifier, so whether the window drawable bled through depended on whether each screen's own root happened to paint a full-size opaque background:

• You tab (MenuScreenContent) — uses CodeScaffold → Material Scaffold(backgroundColor = CodeTheme.colors.background) → opaque; "F" hidden.
• My Account (MyAccountScreen) — bare Column(fillMaxSize) with no background modifier → transparent; "F" bleeds through.
• Wallet (WalletScreen) — bare Column / LazyColumn with no background → transparent center; "F" bleeds through.
• Chat tab (MessengerScreen) — paints its own opaque background; "F" hidden.

The logo is the window drawable (behind Compose), not a Compose element — navigator.replaceAll and all nav transitions have no effect on it.

Fix

Paint CodeTheme.colors.background (#19191A, identical to splash_background) on the single outermost root Box in App.kt that wraps both NewAppContent and AppContent. One change covers all screens on both nav paths.

-Box(modifier = semanticsModifier) {
+Box(modifier = semanticsModifier.fillMaxSize().background(CodeTheme.colors.background)) {

The cold-start splash (window background visible before the first Compose frame) is unchanged — the fix only prevents bleed-through after Compose takes the surface.

Safety

• Haze/blur (hazeSource on the inner Box in NewAppContent): unaffected. Haze only samples Compose-layer pixels; its source node is a child of this root Box, not behind it. The backdrop color used by the nav bar blur (CodeTheme.colors.background) is unchanged.
• Nav3 OverlayScene / bottom-sheets: unaffected. Their transparency is inter-Compose-layer (showing the screen behind them in the Compose hierarchy), which this change does not touch. The opaque root Box is below the entire NavDisplay.
• No screen intentionally relies on seeing the window drawable: confirmed by searching all Kotlin source for R.drawable.splash, ic_launcher_foreground, windowIsTranslucent, and translucent-theme patterns — zero hits in non-build-artifact files.