Folder sharing, shared ChatGPT subscription, standalone demo deployment

[tim4431]

• Server AI can use a ChatGPT subscription: an admin signs in under Settings → Server → Shared AI provider (/api/admin/ai-providers/chatgpt/*); tokens stored encrypted, refreshed once per shared entry under its own lock, the signed-in e-mail shown to admins only, metered by the shared allowance like a key.
• Folder sharing.
• The public demo is its own compose project (cloud/deploy/demo/, /root/Container/gamma-demo on the VPS), reached by the account project's Caddy over the external gamma-edge network; the image tag is GAMMA_TAG in the demo's .env. update-demo-server rewritten for it; update-account-server notes the network.
• gammapdf.com points to the demo: hero "Try the demo", header Demo link, /demo short link; README link row.
• Keyboard shortcuts improved, unused tests trimmed.

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