fix: single branded splash (invisible system icon), December title, first-open lag

- System splash now uses a transparent icon, so there is no system icon that
  shrinks into the in-app splash: just black, then the branded screen (icon +
  Calendarr name + copyright), held until data is loaded
- Month/quarter title uses TextStyle.FULL (format style) with a hard fallback;
  FULL_STANDALONE had the same desugar bug as the LLLL pattern (December showed
  only the year)
- First open: load the full +/- cacheMonths window behind the splash and only
  then reveal, so the app no longer enters mid-load and stutters

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

app/src/main/res/values/themes.xml

@@ -9,10 +9,11 @@
         <item name="android:windowLightStatusBar">false</item>
     </style>
 
-    <!-- System splash (core-splashscreen); covers the window from frame 0. -->
+    <!-- System splash (core-splashscreen): plain black with an invisible icon,
+         so the only branded screen the user sees is the in-app splash. -->
     <style name="Theme.Calendarr.Starting" parent="Theme.SplashScreen">
         <item name="windowSplashScreenBackground">@android:color/black</item>
-        <item name="windowSplashScreenAnimatedIcon">@drawable/splash_icon</item>
+        <item name="windowSplashScreenAnimatedIcon">@drawable/splash_transparent</item>
         <item name="postSplashScreenTheme">@style/Theme.Calendarr</item>
     </style>
 </resources>