Refactor: one Drive access type #82

What to build

Decide whether to collapse DriveAccess (Destinations) and DestinationAccess (Recording) into one type.

Both carry (folder, release). App/RecordScreen.swift converts one to the other with a closure wrapper. Their release closures differ in isolation (@Sendable vs @MainActor), which is the real question: pick one that works for both the security-scoped Drive folder and the Recorder's use, then delete the wrapper.

Speculative, so settle the design first (which module owns the type, what isolation release has) before changing code.

Also look at Show.useDrive/joinDrive, which leave driveFolder as mutable state the Recorder also changes around. Note anything worth tightening.

Found in the architecture review after #13.

Acceptance criteria

• A short decision comment on this issue (one type or two, and why) before any code
• If merged: one access type, no conversion closure in the App, ./scripts/verify.sh passes

Blocked by

None.