fix: always apply aresample sync filter and tighten audio sync tolerances

- Always route audio through aresample=async=1:first_pts=0 filter during
  muxing on both macOS and Windows, even when duration delta is small.
  Previously skipped when delta ≤50ms, which left progressive clock drift
  (from CPU load) completely uncorrected.
- Lower sync detection threshold from 50ms to 20ms so tempo correction
  kicks in earlier.
- Use explicit 48kHz AudioContext sample rate in both browser recording
  and export rendering to prevent sample rate mismatch drift.
- Reduce MediaRecorder timeslice from 1000ms to 250ms to reduce chunk
  loss under CPU pressure.
- Tighten export audio sync: seek threshold 300ms→150ms, tolerance
  15ms→8ms, correction window 2s→0.5s, max adjustment ±8%→±12%.
- Lower companion audio start delay threshold from 50ms to 25ms.

src/hooks/useScreenRecorder.ts

@@ -18,7 +18,7 @@ const HIGH_FRAME_RATE_BOOST = 1.7;
 const DEFAULT_WIDTH = 1920;
 const DEFAULT_HEIGHT = 1080;
 const CODEC_ALIGNMENT = 2;
-const RECORDER_TIMESLICE_MS = 1000;
+const RECORDER_TIMESLICE_MS = 250;
 const BITS_PER_MEGABIT = 1_000_000;
 const MIN_FRAME_RATE = 30;
 const CHROME_MEDIA_SOURCE = "desktop";
@@ -1090,7 +1090,7 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 				const micAudioTrack = microphoneStream.current?.getAudioTracks()[0];
 
 				if (systemAudioTrack && micAudioTrack) {
-					const context = new AudioContext();
+					const context = new AudioContext({ sampleRate: 48000 });
 					mixingContext.current = context;
 					const systemSource = context.createMediaStreamSource(
 						new MediaStream([systemAudioTrack]),

src/lib/exporter/audioEncoder.ts

@@ -555,7 +555,7 @@ export class AudioProcessor {
 				throw new Error("Export cancelled");
 			}
 
-			audioContext = new AudioContext();
+			audioContext = new AudioContext({ sampleRate: 48000 });
 			const currentDestinationNode = audioContext.createMediaStreamDestination();
 			destinationNode = currentDestinationNode;
 
@@ -721,7 +721,7 @@ export class AudioProcessor {
 							continue;
 						}
 
-						if (Math.abs(audioEl.currentTime - targetTimeSec) > 0.3) {
+						if (Math.abs(audioEl.currentTime - targetTimeSec) > 0.15) {
 							audioEl.currentTime = targetTimeSec;
 						}
 
@@ -729,6 +729,9 @@ export class AudioProcessor {
 							basePlaybackRate: playbackRate,
 							currentTime: audioEl.currentTime,
 							targetTime: targetTimeSec,
+							toleranceSeconds: 0.008,
+							correctionWindowSeconds: 0.5,
+							maxAdjustment: 0.12,
 						});
 						if (Math.abs(audioEl.playbackRate - syncedPlaybackRate) > 0.0001) {
 							audioEl.playbackRate = syncedPlaybackRate;
@@ -748,7 +751,7 @@ export class AudioProcessor {
 
 						if (isInRegion) {
 							const audioOffset = (currentTimeMs - region.startMs) / 1000;
-							if (Math.abs(audioEl.currentTime - audioOffset) > 0.3) {
+							if (Math.abs(audioEl.currentTime - audioOffset) > 0.15) {
 								audioEl.currentTime = audioOffset;
 							}
 
@@ -756,6 +759,9 @@ export class AudioProcessor {
 								basePlaybackRate: playbackRate,
 								currentTime: audioEl.currentTime,
 								targetTime: audioOffset,
+								toleranceSeconds: 0.008,
+								correctionWindowSeconds: 0.5,
+								maxAdjustment: 0.12,
 							});
 							if (Math.abs(audioEl.playbackRate - syncedPlaybackRate) > 0.0001) {
 								audioEl.playbackRate = syncedPlaybackRate;

src/lib/mediaTiming.test.ts

@@ -23,10 +23,11 @@ describe("clampMediaTimeToDuration", () => {
 describe("estimateCompanionAudioStartDelaySeconds", () => {
 	it("returns the positive tail gap when companion audio is shorter", () => {
 		expect(estimateCompanionAudioStartDelaySeconds(10, 9.6)).toBeCloseTo(0.4);
+		expect(estimateCompanionAudioStartDelaySeconds(10, 9.97)).toBeCloseTo(0.03);
 	});
 
 	it("ignores tiny or negative differences", () => {
-		expect(estimateCompanionAudioStartDelaySeconds(10, 9.97)).toBe(0);
+		expect(estimateCompanionAudioStartDelaySeconds(10, 9.99)).toBe(0);
 		expect(estimateCompanionAudioStartDelaySeconds(10, 10.5)).toBe(0);
 	});
 });

src/lib/mediaTiming.ts

@@ -19,7 +19,7 @@ export function estimateCompanionAudioStartDelaySeconds(
 	const safeAudioDuration = Math.max(0, audioDuration ?? 0);
 	const estimatedDelaySeconds = safeTimelineDuration - safeAudioDuration;
 
-	return estimatedDelaySeconds > 0.05 ? estimatedDelaySeconds : 0;
+	return estimatedDelaySeconds > 0.025 ? estimatedDelaySeconds : 0;
 }
 
 export function getMediaSyncPlaybackRate({