Serve .ipuz downloads as application/octet-stream

[fokality]

Fixes #603

Problem

Downloading a puzzle as .ipuz on Android produces puzzle.ipuz.json. /api/ipuz/[puzzleId] responds with Content-Type: application/json, and Android appends .json to the filename for that type.

Change

Send the response as application/octet-stream so the .ipuz extension is preserved. Content-Disposition and the other headers are unchanged.

Testing

• Verified the bug on an Android phone against a local dev instance (got .ipuz.json), and verified the fix on the same device (now .ipuz).
• Added app/__tests__/ipuzApi.test.ts, which calls the route handler with getPuzzle/exportIpuz mocked and asserts the Content-Type and Content-Disposition headers. It fails without the fix.
• Existing converter tests still pass; eslint, tsc --noEmit and prettier are clean on the changed files.

This adds the first API route test, using jest.unstable_mockModule. Happy to add similar header tests for the puz and pdf routes in a follow-up if that's useful.

🤖 Generated with Claude Code

[fokality]

Note on alternatives: I also tried switching Content-Disposition from inline to attachment while keeping Content-Type: application/json. On Android the download was still named .ipuz.json, so the content type is what causes the extension to be appended. That's why this PR changes Content-Type rather than the disposition.