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/data/StartupState.kt

@@ -0,0 +1,15 @@
+package com.scarriffle.calendarr.data
+
+import kotlinx.coroutines.flow.MutableStateFlow
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Shared startup readiness flag used to keep the system splash screen on screen
+ * until the first events have loaded (so the app never appears mid-load).
+ */
+@Singleton
+class StartupState @Inject constructor() {
+    val ready = MutableStateFlow(false)
+    fun markReady() { ready.value = true }
+}