fix(recording): finalize native Windows recording without renderer round-trip

[ThairaHub]

Closes #11 (harvested from archived upstream siddharthvaddem#689).

Problem

finalizeNativeWindowsRecording read the entire on-disk screen recording back into renderer memory via readBinaryFile(nativeScreenPath) and shipped those bytes over IPC to store-recorded-session — which then wrote the same bytes back to the same path (resolveRecordingOutputPath(fileName) resolves to where the native helper already wrote the file). For long recordings that's a multi-GB read + IPC marshal + redundant rewrite, with renderer OOM risk on stop — the exact failure mode siddharthvaddem#687 fixed for the webcam sidecar.

The round-trip existed only to attach the webcam sidecar: stop-native-windows-recording already writes cursor telemetry, the session, and the manifest in the main process.

Fix

• Generalize the macOS attach-native-mac-webcam-recording handler into a platform-agnostic attach-webcam-to-screen-recording (it was always pure disk plumbing; the darwin guard was the only mac-specific part). Error strings made platform-neutral; streamed-webcam rollback behavior unchanged.
• Windows finalize now calls it with screenVideoPath + the small webcam buffer (empty when the webcam streamed to disk per fix(recording): stream native-capture webcam to disk to prevent OOM crash on stop siddharthvaddem/openscreen#687). The screen file never leaves disk.
• macOS finalize migrates to the same channel — one handler, two platforms; old channel removed (no remaining references).
• Webcam attach failure now surfaces a toast (parity with macOS) instead of silently dropping the webcam; the screen-only session from the stop handler still stands.

Acceptance criteria (from #11)

• ✅ readBinaryFile(nativeScreenPath) removed from the renderer finalize path
• ✅ Main-process handler owns the screen + webcam disk merge/patch
• ✅ Renderer only marshals small in-memory webcam data (if any)
• ✅ tsc --noEmit 0 errors, biome check clean, vitest 225/225, browser tests 6/6, vite build OK

Note: like upstream's siddharthvaddem#687, the native Windows/macOS capture finalize can't be exercised in unit tests (needs the platform helper + real camera). The renderer↔main streaming contract is covered by the existing recorderHandle/recordingStream tests.

🤖 Generated with Claude Code

[mepsd]

is this available in release build?