feat: add core rive service and canvas directive

[denisyilmaz]

Summary

• introduce DI tokens for configuring Rive asset loading
• add RiveService with signal-based state and asset fetching
• implement RiveCanvas directive using signals and effects for artboard rendering
• rename canvas directive for clearer naming and remove generated scaffolding

Testing

• npm install --legacy-peer-deps (fails: tunneling socket could not be established, statusCode=403)
• npx nx test ng-rive --watch=false (fails: Could not find Nx modules at "/workspace/ng-rive"; have you run npm/yarn install?)

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https://chatgpt.com/codex/tasks/task_b_689b6cc2aa188321b6f2c8a5e8a5b33e

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Latest commit: 871a619

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