ux: rename 'Native' aspect ratio label to 'Original'

The aspect ratio dropdown showed 'Native', which is video-industry jargon
that isn't self-explanatory for most users. Renaming it to 'Original'
makes it immediately clear that this option preserves the source video's
own dimensions.

The internal `"native"` value in the AspectRatio union type is unchanged;
only the display string returned by `getAspectRatioLabel()` is updated.

Closes #607

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

src/utils/aspectRatioUtils.ts

@@ -63,7 +63,7 @@ export function getAspectRatioDimensions(
 }
 
 export function getAspectRatioLabel(aspectRatio: AspectRatio): string {
-	if (aspectRatio === "native") return "Native";
+	if (aspectRatio === "native") return "Original";
 	return aspectRatio;
 }