feat: Add runtime animation replacement

[antonstefer]

Summary

• add runtime animation replacement methods so animations can be swapped without recreating the platform view
• upgrade the native Lottie and Android/iOS build toolchains used by the plugin and example app
• harden native dynamic value updates so malformed color or opacity inputs return platform errors instead of crashing

Changes

• Dart API: add setAnimationFromUrl(), setAnimationFromAsset(), and setAnimationFromJson() to LottieController
• Dart cleanup: replace deprecated Color.value with toARGB32() and make StandardMessageCodec const in LottieView
• Android native: add runtime animation swap handlers, enable async updates, move the failure listener into init, and validate setValue arguments and payload parsing
• iOS native: add runtime animation swap handlers and validate setValue arguments and payload parsing, including locale-safe opacity parsing
• Example app: add buttons to swap animations at runtime, log animation state changes, and update the iOS example app setup for the current Flutter embedding
• Build updates: upgrade Kotlin to 2.1.0, AGP to 8.7.0, Java to 17, appcompat to 1.7.1, lottie-android to 6.7.1, and lottie-ios to 4.6.0; migrate the example Android Gradle setup to the modern Flutter plugin configuration
• Docs: refresh CHANGELOG.md and repository guidance in CLAUDE.md