Setup release infrastructure for v1.0.0

[OneXeor]

Summary

This PR sets up the complete release infrastructure for publishing UXCam-KMP to Maven Central.

Changes

Release Infrastructure:

• Add Vanniktech Maven Publish Plugin (v0.30.0) for Maven Central publishing
• Configure Sonatype Central Portal as the publishing target
• Create automated release workflow triggered on tag push (v*)
• Centralize version management in gradle.properties (VERSION_NAME=1.0.0)

Documentation:

• Add CHANGELOG.md following Keep a Changelog format
• Add CODE_OF_CONDUCT.md (referenced in CONTRIBUTING.md)
• Add KMP installation instructions to README

Code Quality:

• Add null safety checks to Android implementation
• Replace System.err with Napier logging
• Fix validate-pr workflow to target 'main' branch (was 'master')

Files Changed

File	Change
build.gradle.kts	Add vanniktech plugin
gradle.properties	Add version and Maven properties
uxcam-kmp-shared/build.gradle.kts	Add publishing config
uxcam-kmp-shared/src/androidMain/.../UXCam.kt	Null checks + Napier
.github/workflows/release.yml	New release workflow
.github/workflows/validate-pr.yml	Fix branch target
CHANGELOG.md	New
CODE_OF_CONDUCT.md	New
README.md	Add KMP installation

Required Secrets (before first release)

• MAVEN_CENTRAL_USERNAME - Sonatype Central Portal token username
• MAVEN_CENTRAL_PASSWORD - Sonatype Central Portal token password
• SIGNING_KEY - GPG private key (ASCII armored)
• SIGNING_PASSWORD - GPG key passphrase

Release Process (after merge)

git checkout main
git pull
git tag v1.0.0
git push origin v1.0.0

Test plan

• Android build passes (./gradlew :uxcam-kmp-shared:compileReleaseKotlinAndroid)
• Verify GitHub secrets are configured
• Test release workflow with tag push