docs: refresh demo.gif with guided tour and theme toggle

[askuma]

Summary

• Re-recorded docs/demo.gif — the old one predated the guided tour and light/dark theme toggle
• New recording: real pip install + real CLI run against fastapi/full-stack-fastapi-template, then a real browser walkthrough of the generated page (tour steps, dark-mode scroll, theme toggle to light, light-mode scroll)

Test plan

• Verified pip install + workflow-generator CLI output in the recorded terminal session
• Verified tour popup, dark/light scroll, and theme toggle render correctly in extracted sanity-check frames