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

src/main/java/net/anomaly/justasplash/JustASplash.java

@@ -0,0 +1,35 @@
+package net.anomaly.justasplash;
+
+import net.fabricmc.api.ClientModInitializer;
+import net.fabricmc.fabric.api.client.event.lifecycle.v1.ClientTickEvents;
+import net.minecraft.registry.Registries;
+import net.minecraft.registry.Registry;
+import net.minecraft.sound.SoundEvent;
+import net.minecraft.util.Identifier;
+import net.anomaly.justasplash.config.SplashConfig;
+import net.anomaly.justasplash.splash.SplashAssetCache;
+import net.anomaly.justasplash.splash.SplashKeybind;
+import net.anomaly.justasplash.splash.SplashManager;
+import net.anomaly.justasplash.splash.SplashSound;
+import net.anomaly.justasplash.util.Compat;
+
+public class JustASplash implements ClientModInitializer {
+    public static final Identifier SPLASH_SOUND_ID = Compat.id("justasplash", "splash");
+    public static SoundEvent SPLASH_SOUND_EVENT;
+
+    @Override
+    public void onInitializeClient() {
+        SPLASH_SOUND_EVENT = SoundEvent.of(SPLASH_SOUND_ID);
+        Registry.register(Registries.SOUND_EVENT, SPLASH_SOUND_ID, SPLASH_SOUND_EVENT);
+
+        SplashConfig.load();
+        SplashAssetCache.preload();
+        SplashKeybind.register();
+        SplashSound.setup();
+
+        ClientTickEvents.END_CLIENT_TICK.register(client -> {
+            if (SplashKeybind.wasPressed()) SplashManager.show();
+            SplashManager.tick();
+        });
+    }
+}