perf: unified splash held until loaded, smoother month scrolling, serialized loads

- Single system splash (core-splashscreen) kept on screen until the first
  events load (StartupState), then reveals the ready app — removes the
  two-stage 'icon then title' splash and the laggy half-loaded entry
- Month scrolling: dropped per-row BoxWithConstraints (measured grid width once
  via onSizeChanged) and resolve bar colours once during packing instead of
  every recomposition → much smoother scroll
- Serialize all event loads behind a Mutex and skip redundant state writes so
  scroll-triggered month loads no longer pile up / thrash the UI

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/java/com/scarriffle/calendarr/MainActivity.kt

@@ -4,16 +4,35 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.core.splashscreen.SplashScreen.Companion.installSplashScreen
+import androidx.lifecycle.lifecycleScope
+import com.scarriffle.calendarr.data.CredentialStore
+import com.scarriffle.calendarr.data.StartupState
 import com.scarriffle.calendarr.ui.CalendarrRoot
 import dagger.hilt.android.AndroidEntryPoint
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import javax.inject.Inject
 
 @AndroidEntryPoint
 class MainActivity : ComponentActivity() {
+
+    @Inject lateinit var startupState: StartupState
+    @Inject lateinit var credentialStore: CredentialStore
+
     override fun onCreate(savedInstanceState: Bundle?) {
-        // Covers the window from the first frame (no warm-start flash), then
-        // hands off to the in-app splash which stays until data is loaded.
-        installSplashScreen()
+        // Single, uniform splash: keep the system splash on screen until the
+        // first events have loaded (or a timeout), so there is no two-stage
+        // splash and no entering a half-loaded, janky app.
+        val splash = installSplashScreen()
+        splash.setKeepOnScreenCondition { !startupState.ready.value }
+
         super.onCreate(savedInstanceState)
+
+        // Nothing to load before login → reveal immediately.
+        if (!credentialStore.isLoggedIn) startupState.markReady()
+        // Safety timeout so the splash can never hang.
+        lifecycleScope.launch { delay(6000); startupState.markReady() }
+
         setContent {
             CalendarrRoot()
         }