Fix security and reliability issues

1. Validate URL scheme in open-external-url handler
   - Prevent opening file:// or other dangerous schemes via shell.openExternal
   - Only allow http:, https:, and mailto: protocols

2. Fix latest video detection using mtime instead of lexicographic sort
   - Lexicographic sort gives wrong results (e.g. recording-9 > recording-10)
   - Now sorts by file modification time for reliable latest-file detection

3. Add null guard for AudioData.format in cloneWithTimestamp
   - Replace non-null assertion (!) with proper validation
   - Throws descriptive error if format is unexpectedly null

4. Prevent encodeQueue counter underflow in VideoExporter
   - Use Math.max(0, ...) to prevent negative queue count

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/exporter/audioEncoder.ts

@@ -459,7 +459,10 @@ export class AudioProcessor {
 	}
 
 	private cloneWithTimestamp(src: AudioData, newTimestamp: number): AudioData {
-		const isPlanar = src.format?.includes("planar") ?? false;
+		if (!src.format) {
+			throw new Error("AudioData format is required for cloning");
+		}
+		const isPlanar = src.format.includes("planar");
 		const numPlanes = isPlanar ? src.numberOfChannels : 1;
 
 		let totalSize = 0;
@@ -476,7 +479,7 @@ export class AudioProcessor {
 		}
 
 		return new AudioData({
-			format: src.format!,
+			format: src.format,
 			sampleRate: src.sampleRate,
 			numberOfFrames: src.numberOfFrames,
 			numberOfChannels: src.numberOfChannels,

src/lib/exporter/videoExporter.ts

@@ -422,7 +422,7 @@ export class VideoExporter {
 				})();
 
 				this.muxingPromises.push(muxingPromise);
-				this.encodeQueue--;
+				this.encodeQueue = Math.max(0, this.encodeQueue - 1);
 			},
 			error: (error) => {
 				console.error("[VideoExporter] Encoder error:", error);