Surface AVAssetWriter failures instead of failing silently

[sfegette]

Summary

The recording pipeline currently stops appending when AVAssetWriter is no longer .writing, but it does not reliably surface writer failure details back to the user.

Why this matters

Disk-full, permission, codec, or output-path failures can result in partial or missing recordings that appear to stop normally. For a capture app, silent save failures are high-severity.

Observed gaps

• startRecording() sets up the writer but does not validate later writer failure states in a user-visible way
• captureOutput(...) returns early when writer.status != .writing
• finishWriting {} does not inspect writer.status or writer.error
• lastError is not updated on finish failure

Suggested direction

• Inspect writer.status and writer.error during recording and during finishWriting
• Surface a clear user-visible error when finalize/save fails
• Consider rejecting stop completion until the writer finishes with success/failure
• Add targeted logging for output path, file type, and writer error details

Relevant code

• Capture/CaptureManager.swift lines 421-481
• Capture/CaptureManager.swift lines 623-654
• Capture/CaptureManager.swift lines 512-523

Acceptance criteria

• Writer failures produce a user-visible error
• Finalization success/failure is known at stop time
• Disk/output failures are diagnosable from both UI and logs