Refactor Next.js demo

[bean1352]

• Scaffold a local PowerSync service (Docker Compose + sync config) so the demo runs fully self-contained
• Replace MUI/Emotion with Tailwind CSS v4
• Add API routes for JWT auth (/api/auth/token, /api/auth/keys) and CRUD writes (/api/data)
• Support POWERSYNC_PRIVATE_KEY/POWERSYNC_PUBLIC_KEY env vars, auto-generate a temporary key pair if not set
• Make root layout a server component with metadata export
• Extract interactive sync UI into a SyncedContent client component

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Latest commit: 6b4e7df

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[khawarizmus]

I am not sure whether we want to commit this logic. It feels more like something to have locally than to add to the official demo.

[bean1352]

I have updated the demo to require users to generate a key pair manually, rather than generating one automatically.