fix: replace real-time audio export with streaming decode + chunked offline rendering

Major rewrite of the audio export pipeline for stability and memory efficiency:

## Streaming Decode
- Primary decode path uses WebDemuxer + AudioDecoder (WebCodecs streaming)
- Falls back to bulk decodeAudioData if streaming fails
- Avoids holding full compressed file + decoded PCM simultaneously

## Chunked Offline Rendering
- Processes timeline in 30-second OfflineAudioContext chunks
- Memory bounded to ~30s of PCM per chunk regardless of recording length

## Chunked WAV Writing
- Writes PCM in ~256KB chunks instead of single massive ArrayBuffer
- Eliminates OOM for long recordings on the native/FFmpeg export path

## Chunked Encoding
- Single AudioEncoder kept alive across all chunks (clean AAC stream)
- Proper backpressure and error propagation per chunk

## scheduleBufferThroughTimeline chunk windowing
- Clips source nodes to chunk boundaries for correct cross-chunk audio
- Backwards compatible defaults when no chunk params provided

## Other export fixes
- Progress cap: 99% -> 100% in both exporters and UI
- Muxing errors: propagated instead of silently swallowed
- Mac recording: enhanced warning when no audio files for muxing
- Export timeout: eliminated by removing real-time rendering

electron/ipc/recording/mac.ts

@@ -158,7 +158,10 @@ export async function muxNativeMacRecordingWithAudio(
 	}
 
 	if (availableAudioInputs.length === 0) {
-		console.warn("[mux] No valid audio files to mux");
+		console.warn(
+			"[mux] No valid audio files to mux — video will have no audio. " +
+				`system=${systemAudioPath ?? "none"} mic=${microphonePath ?? "none"}`,
+		);
 		return;
 	}
 

src/components/video-editor/VideoEditor.tsx

@@ -4177,8 +4177,8 @@ export default function VideoEditor() {
 		? Math.min(
 				typeof exportProgress?.renderProgress === "number"
 					? exportProgress.renderProgress
-					: (exportProgress?.percentage ?? 99),
-				99,
+					: (exportProgress?.percentage ?? 100),
+				100,
 			)
 		: null;
 	const isLightningExportInProgress =
@@ -4454,8 +4454,7 @@ export default function VideoEditor() {
 									</p>
 									{isRenderingAudio ? (
 										<p className="mt-1 text-[11px] text-muted-foreground/70">
-											Audio requires real-time playback for speed/overlay
-											edits
+											Processing audio with speed/overlay edits
 										</p>
 									) : exportRenderSpeedLabel ? (
 										<p className="mt-1 text-[11px] text-muted-foreground/70">

src/lib/exporter/modernVideoExporter.ts

@@ -1152,10 +1152,10 @@ export class ModernVideoExporter {
 		const estimatedTimeRemaining =
 			averageRenderFps > 0 ? remainingFrames / averageRenderFps : 0;
 		const safeRenderProgress =
-			phase === "finalizing" ? Math.max(0, Math.min(renderProgress ?? 99, 99)) : undefined;
+			phase === "finalizing" ? Math.max(0, Math.min(renderProgress ?? 100, 100)) : undefined;
 		const percentage =
 			phase === "finalizing"
-				? (safeRenderProgress ?? 99)
+				? (safeRenderProgress ?? 100)
 				: totalFrames > 0
 					? (currentFrame / totalFrames) * 100
 					: 100;
@@ -1399,6 +1399,10 @@ export class ModernVideoExporter {
 						}
 					} catch (error) {
 						console.error("Muxing error:", error);
+						if (!this.encoderError) {
+							this.encoderError = error instanceof Error ? error : new Error(String(error));
+						}
+						this.cancelled = true;
 					}
 				});
 				this.encodeQueue--;

src/lib/exporter/videoExporter.ts

@@ -885,10 +885,10 @@ export class VideoExporter {
 		const estimatedTimeRemaining =
 			averageRenderFps > 0 ? remainingFrames / averageRenderFps : 0;
 		const safeRenderProgress =
-			phase === "finalizing" ? Math.max(0, Math.min(renderProgress ?? 99, 99)) : undefined;
+			phase === "finalizing" ? Math.max(0, Math.min(renderProgress ?? 100, 100)) : undefined;
 		const percentage =
 			phase === "finalizing"
-				? (safeRenderProgress ?? 99)
+				? (safeRenderProgress ?? 100)
 				: totalFrames > 0
 					? (currentFrame / totalFrames) * 100
 					: 100;