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

gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,7 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.12-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists