Update Next.js tutorial and SDK docs for SSR support and Turbopack compatibility

[benitav]

TODO: release https://github.com/powersync-ja/powersync-js/pull/815/changes

• Added note clarifying PowerSync is designed for client-side applications and won't provide data during SSR (SDK gracefully handles Node.js environments without throwing errors)
• Updated tutorial to recommend Turbopack for Next.js 16+
• Simplified setup by removing DynamicSystemProvider workaround - users can now use SystemProvider directly
• Added powersync-web copy-assets postinstall command for copying worker files
• Moved legacy Webpack configuration to a separate section for older Next.js versions
• Updated SystemProvider example with new worker configuration pattern