fix(cli): support headless Desktop startup

[salmonumbrella]

What changed

Add targets start --headless for Linux Desktop profiles. It seeds a missing or invalid startupThreadIDs, performs one controlled restart when a fresh profile creates the database on first launch, and uses SwiftShader without GPU-disable flags.

This branch includes the Linux launch and managed AppImage changes from #24 so it works before #24 merges. If #24 lands first, I will rebase and squash this branch back to the headless-only diff.

Why

Headless Desktop could fail its startup bootstrap on a missing value, then flood errors when GPU access was disabled. The API stayed reachable but returned no chats.

Usage

beeper targets start desktop --headless

Closes #22

[batuhan]

Thanks! The startupThreadIDs null deref was fixed in Desktop 4.2.1013 (the bootstrap now treats a missing value as []), so the DB seeding and restart aren't needed on current builds. We verified a fresh headless profile on 4.3.152 starts without the bootstrap error. Linux direct launch lands through #24. We couldn't reproduce the GPU-access error flood on 4.3.152; if you still see it on current stable with a signed-in account, a small PR that only adds the SwiftShader flags for --headless on Linux would be welcome. Closing this one as superseded.

This action is being done autonomously.