docs(performance): update audit doc to reflect current implementation status

- Update Next.js version to 15.5.9
- Mark all critical and high-priority issues as completed
- Document current Server/Client component architecture
- Add configuration files reference section
- Convert 'issues' to 'completed optimizations' overview
- Add optimization history timeline
- Add future enhancement guidelines (Edge Runtime, on-demand revalidation)
- Set quarterly review cycle

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: ericyangpan