fix: fail cleanly on empty manual track selection (closes #173)

[negativeeddy]

Automated Fix

Fixes #173

Problem

Deselecting all tracks in Manual Selection mode submits ManualSelectionTrackNumbers = "[]" (a non-empty string), so the apply block runs, sets every Process=false, and the rip loop has zero eligible tracks. The post-rip check then throws the confusing "MakeMKV rip produced no ripped tracks" error and the job fails.

Changes

• API (ApiController.SubmitManualSelection): reject an empty trackNumbers array with a clear error. The job stays parked in ManualSelectionStarted so the user can correct the selection and resubmit (or cancel the job).
• Pipeline (ArmRipperService): defensive check — if an empty selection somehow reaches the pipeline, fail the job cleanly with "No tracks selected" instead of the confusing downstream error.
• UI (JobDetail.cshtml): client-side guard in submitManualSelection() that alerts immediately when no tracks are checked, avoiding a round-trip.

Testing

• New test ManualSelection_EmptySelection_FailsJobCleanly (pipeline fails cleanly, no rip runs)
• New test SubmitManualSelection_EmptySelection_RejectsAndKeepsJobWaiting (API rejects, job stays waiting)
• All existing tests pass (Core: 294, WebUi: 80)

[negativeeddy]

Reviewed and approved. The fix matches the issue's proposed fix: the API rejects empty selections (job stays parked for correction), the pipeline fails cleanly with a clear 'No tracks selected' message as a defensive guard, and the UI alerts client-side. Build passes (0 warnings/errors); all tests pass (Core 294, WebUi 80) including two new regression tests. Ready for merge.