Export and import a project file

[ankitsejwal]

Closes #40.

Now you can Save your work to a single file and Open it later or on another machine — captions, crop, orientation, settings, and the images themselves, all self-contained.

How it works

• Save serializes the photos (images embedded as base64 data URLs) + the settings snapshot into one polaroid-project.json.
• Open parses it back into live photos with fresh ids and object URLs, replaces the working set, and restores settings. Invalid files are rejected with a message.
• Pure client-side — no account, no cloud, no upload. Just a file you own. (Open works even with an empty canvas; Save is enabled once you have photos.)
• Opening a project flows through the existing persistence, so it's also written to IndexedDB.

Verification
14/14 headless assertions on a full export→import round-trip: captions, place, takenAt, camera line, crop, orientation and settings all survive, image bytes are byte-identical, new ids are assigned, and malformed files throw. typecheck / oxlint / build green.