Add OOTT-branded native splash for Android and iOS

Replace the vanilla Flutter splash with the OOTT wordmark (Barlow
Condensed Bold) on the Gruvbox-dark background, matching the in-app
AppBar badge. Generated via flutter_native_splash, covering legacy
Android, the Android 12+ SplashScreen API, and iOS (light + dark).

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

frontend/ios/Runner/Assets.xcassets/LaunchBackground.imageset/Contents.json

@@ -0,0 +1,22 @@
+{
+  "images" : [
+    {
+      "filename" : "background.png",
+      "idiom" : "universal"
+    },
+    {
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "dark"
+        }
+      ],
+      "filename" : "darkbackground.png",
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}