Work exists in one browser only, with no way to save or move it #420

A site someone builds here exists in exactly one browser, on one device, and there is no way to get it out or back in.

Walked the full flow on production today. Gallery, Use, editor, change a heading, Export: it works, 76 KB of files, the exported page paints and scrubs on desktop and phone with no console errors. But the editable document lives only in this browser's IndexedDB. Clear browsing data and it is gone. Switch laptop and it is gone. Hand it to a colleague and you cannot. The editor already knows this, which is why it has to say "that background is no longer in this browser" (editor/page.tsx:309).

Someone who spends an hour on a site and loses it does not come back, and does not recommend us. This is the difference between a toy and a tool.

Two buttons:

• Download project file. One .json with the sections, theme, style recipe and signup settings. Small, because the frames are a recipe rather than pixels.
• Open project file. Reads it back, in any browser, on any machine.

That also gives a developer something to commit to git, and gives anyone a backup that does not depend on us staying online.

Done when: a project exported in one browser profile opens in a different one with the same sections, theme and background, a test covers the round trip, and the editor says plainly where the work is stored before someone loses it.