fix(android, tabs): broaden selectedTab fallback to cover applyDayNightUiMode race

The original patch guarded `updateBottomNavigationViewAppearance` and made
the lookup helpers null-safe, but field testing surfaced a SECOND crash
path that bypasses those guards:

  IllegalStateException: [RNScreens] No selected tab present
    at TabsContainer.getSelectedTab(TabsContainer.kt:88)
    at TabsAppearanceCoordinator.updateTabAppearance(TabsAppearanceCoordinator.kt:21)
    at TabsContainer.applyDayNightUiMode(TabsContainer.kt:591)
    at TabsContainer.onAttachedToWindow$lambda$11(TabsContainer.kt:271)
    at ColorSchemeCoordinator.applyResolvedColorScheme(ColorSchemeCoordinator.kt:108)
    at ColorSchemeCoordinator.setup$react_native_screens_release(ColorSchemeCoordinator.kt:73)
    at TabsContainer.onAttachedToWindow(TabsContainer.kt:270)

ColorSchemeCoordinator.setup fires the day/night callback as part of
the same onAttachedToWindow window where fragment screenKeys haven't
flushed. The callback reaches updateTabAppearance → selectedTab on a
code path that did not previously touch updateBottomNavigationViewAppearance,
so guarding only that method left this crash unmitigated.

Guard at the source instead: make the `selectedTab` getter fall back
to the first bound fragment when navState.selectedScreenKey does not
resolve. Only throw when no usable fragment exists at all — that is the
genuine programmer error the original `checkNotNull` was meant to
catch.

Semantics:
  - Happy path (fragments bound) — unchanged: getFragmentForScreenKey
    returns the matching fragment.
  - Initial-mount race (selectedScreenKey set, no fragment bound yet) —
    fallback to first bound fragment; one frame of marginally-wrong
    appearance is far cheaper than a cold-start IllegalStateException
    that kills the activity.
  - Genuine empty state (tabsModel has zero entries) — checkNotNull
    still throws with the same diagnostic message.

Verified on Pixel 9 (Android 16) cold-start with the same minimal
NativeTabs reproduction as the original patch: previously crashed
100% of the time with 'No selected tab present', now starts cleanly.

Author: blackshrub

android/src/main/java/com/swmansion/rnscreens/gamma/tabs/container/TabsContainer.kt

@@ -84,8 +84,19 @@ class TabsContainer internal constructor(
     internal var rejectStaleNavigationStateUpdates: Boolean = false
 
     internal val selectedTab: TabsScreenFragment
+        // During the Fabric initial-mount race, no fragment has bound
+        // its screenKey yet so the strict lookup throws. Fall back to
+        // the first bound fragment; only throw if tabsModel is truly
+        // empty (genuine programmer error vs Fabric prop-batching
+        // ordering). Without this, applyDayNightUiMode →
+        // updateTabAppearance → selectedTab crashes onAttachedToWindow
+        // even when other appearance entry points are guarded, because
+        // ColorSchemeCoordinator invokes the callback on a different
+        // code path that bypasses the entry guards.
         get() =
-            checkNotNull(getFragmentForScreenKey(navState.selectedScreenKey)) { "[RNScreens] No selected tab present" }
+            getFragmentForScreenKey(navState.selectedScreenKey)
+                ?: tabsModel.firstOrNull { !it.tabsScreen.screenKey.isNullOrBlank() }
+                ?: checkNotNull(null as TabsScreenFragment?) { "[RNScreens] No selected tab present" }
 
     internal val invalidationFlags = TabsContainerInvalidationFlags()