Add Maven Central publishing

[OneXeor]

Summary

• Replace GitHub Packages with Maven Central (Sonatype Central Portal)
• Add com.vanniktech.maven.publish plugin
• Simplify README installation (no auth required for consumers)
• Add Maven Central badge

Changes

• Removed GitHub Packages repository and credentials
• Added POM metadata to gradle.properties
• Simplified dependency declaration in README

Before

// Required GitHub Packages auth
maven {
    url = uri("https://maven.pkg.github.com/OneXeor/KDownloader")
    credentials { ... }
}
implementation("dev.onexeor:kdownloader:1.0.0")

After

// Just works with mavenCentral()
implementation("dev.onexeor:kdownloader:1.0.0")

Test plan

• Build passes
• Publishing configuration is correct