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

build.gradle

@@ -0,0 +1,58 @@
+plugins {
+    id "fabric-loom" version "1.9.1"
+    id "maven-publish"
+}
+
+def mcVersion = project.findProperty("mcVersion") ?: "1.21.1"
+
+def ver = [
+    "1.20.1": [mappings: "1.20.1+build.10",  loader: "0.15.11", fabricApi: "0.92.0+1.20.1"],
+    "1.21":   [mappings: "1.21+build.8",     loader: "0.15.11", fabricApi: "0.100.0+1.21"],
+    "1.21.1": [mappings: "1.21.1+build.3",   loader: "0.15.11", fabricApi: "0.101.2+1.21.1"],
+]
+
+def v = ver[mcVersion]
+if (v == null) throw new GradleException("Unsupported MC version: $mcVersion")
+
+version = "${project.findProperty("modVersion") ?: "1.0.0"}+mc${mcVersion}"
+group = project.findProperty("mavenGroup") ?: "net.anomaly"
+
+base {
+    archivesName = "JustASplash"
+}
+
+repositories {
+    mavenCentral()
+    maven { url "https://maven.fabricmc.net/" }
+
+}
+
+dependencies {
+    minecraft "com.mojang:minecraft:${mcVersion}"
+    mappings "net.fabricmc:yarn:${v.mappings}:v2"
+    modImplementation "net.fabricmc:fabric-loader:${v.loader}"
+    modImplementation "net.fabricmc.fabric-api:fabric-api:${v.fabricApi}"
+
+    modImplementation("com.github.usefulness:webp-imageio:0.10.2")
+    include("com.github.usefulness:webp-imageio:0.10.2")
+}
+
+loom {
+    accessWidenerPath = file("src/main/resources/justasplash.accesswidener")
+}
+
+processResources {
+    inputs.property "version", project.version
+    filesMatching("fabric.mod.json") {
+        expand "version": project.version
+    }
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_17
+    targetCompatibility = JavaVersion.VERSION_17
+}
+
+tasks.withType(JavaCompile).configureEach {
+    it.options.release = 17
+}