docs: refresh demo gif (Send to agent, palette accordion)

[pshenok]

Summary

• Re-records docs/demo.gif against the current UI: "Send to agent" button (was "Apply") and the palette accordion (AWS open by default, GCP/Azure/Cloudflare collapsed).
• Same storyboard as before: graph settles, a live tfstate update adds a node, node click shows the inspector, palette accordion browsed, a draft resource gets a wish typed in, then "Send to agent" fires and the confirmation notice shows.

Test plan

• Built packages/ui via build:pkg and served the fixture state through stackcanvas serve with telemetry consent pre-denied (no banner in the recording)
• Recorded via Playwright chromium (1280x800, ~17s) and converted with ffmpeg two-pass palette (960x600, 12fps, 3.54MB, well under the 6MB budget)
• Spot-checked extracted frames: "Send to agent" button, GCP accordion pack open, live-added S3 bucket node, dashed RDS draft with the wish text, and the "Sent to agent" notice