Add a one-click sample project to the public demo

[SLP-DEV1]

Summary

Turns the GitHub Pages demo into an instant try-before-upload experience.

Changes

• add original, MIT-licensed sample mockup and design assets under public/examples/
• add a prominent Try sample project action to the demo banner
• load sample assets through the same FileDrop/onFiles path as normal user uploads
• add Use my files to clear the bundled sample again
• resolve all sample URLs through import.meta.env.BASE_URL so the Pages repository subpath works
• add tests for Pages subpath URL construction
• keep the demo fully client-side with no upload backend

Closes #17