Add project sources and fix .gitignore for Gradle build

- .gitignore: exclude build/, .gradle/, IDE files, OS files
- Gradle 8.12 multi-version build system (Fabric Loom 1.9.1)
- Full source: JustASplash entrypoint, config, asset cache, keybind, renderer, sound, image loaders (PNG/WebP), WindowMixin
- Resources: fabric.mod.json, mixins, localization (en/ru), default splash.png + sounds, icon
- Refactored from me.sashegdev to net.anomaly.justasplash package

settings.gradle

@@ -0,0 +1,10 @@
+pluginManagement {
+    repositories {
+        mavenCentral()
+        maven { url "https://maven.fabricmc.net/" }
+        maven { url "https://maven.quiltmc.org/repository/release/" }
+        gradlePluginPortal()
+    }
+}
+
+rootProject.name = "JustASplash"