fix(audio): handle rejected capture stop flush

[ANIRUDDHA ADAK (aniruddhaadak80)]

Summary

Handle a rejected final audio chunk flush in the capture preload and surface it to the recorder.

Why

MediaRecorder.onstop waited on sendChain without a rejection handler. A failed final chunk flush could become an unhandled rejection and never tell the main process why the segment failed.

Changes

• Extract stop finalization into a testable CommonJS helper.
• On flush rejection, clean up and send audio:error followed by audio:stopped.
• Copy the helper with the other static preload assets and register the regression tests in npm test.
• Cover both rejected and successful flush paths.

Testing

• npm test (existing suite plus the new regression test; not executed yet)
• npm run build (existing CI command; not executed yet)

Local execution was not performed because this contribution is remote-only; no local checkout or source was used.

The new test is unit-level; it does not replace the full Electron/MediaRecorder matrix run by CI.

CI status

• The first Non-Windows and Windows Actions runs are action_required with no jobs or logs; upstream workflow approval is required before the commands can run.
• license/cla was queued at the last check.
• No npm result is claimed until an Actions run executes.

Fixes #15

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