fix(downloads): stop repeating an extension the name already carries

A voice comment's display name is the generated file name, extension
included, so appending the extension again downloaded it as
<uuid>.webm.webm. Both download paths, the single asset route and the
project zip, now append only when the name does not already end in it.

lib/project-download.ts

@@ -4,6 +4,7 @@ import {
   extractImageFileNameFromProxyUrl,
   extractVideoFileNameFromProxyUrl,
   sanitizeAssetDisplayName,
+  withFileExtension,
 } from '@/lib/video-assets';
 
 const DEFAULT_MAX_FILES = 250;
@@ -171,21 +172,21 @@ function buildAssetFileName(videoIndex: number, videoTitle: string, asset: Asset
 
   if (asset.provider === VideoAssetProvider.R2_IMAGE) {
     const fileName = extractImageFileNameFromProxyUrl(asset.sourceUrl);
-    return `${stem}${extensionFromUrl(fileName ?? '', '.png')}`;
+    return withFileExtension(stem, extensionFromUrl(fileName ?? '', '.png'));
   }
   if (asset.provider === VideoAssetProvider.R2_AUDIO) {
     const fileName = extractAudioFileNameFromProxyUrl(asset.sourceUrl);
-    return `${stem}${extensionFromUrl(fileName ?? '', '.webm')}`;
+    return withFileExtension(stem, extensionFromUrl(fileName ?? '', '.webm'));
   }
   if (asset.provider === VideoAssetProvider.R2_VIDEO) {
     const fileName = extractVideoFileNameFromProxyUrl(asset.sourceUrl);
-    return `${stem}${extensionFromUrl(fileName ?? '', '.mp4')}`;
+    return withFileExtension(stem, extensionFromUrl(fileName ?? '', '.mp4'));
   }
   if (asset.provider === VideoAssetProvider.BUNNY) {
-    return `${stem}.mp4`;
+    return withFileExtension(stem, '.mp4');
   }
 
-  return `${stem}.bin`;
+  return withFileExtension(stem, '.bin');
 }
 
 function versionDownloadUrl(version: VersionRow): string | null {

lib/video-assets.ts

@@ -66,6 +66,14 @@ export function sanitizeAssetDisplayName(
   return normalized.slice(0, 200);
 }
 
+/**
+ * A voice comment's display name is the generated file name, extension and all,
+ * so blindly appending the extension named the download `<uuid>.webm.webm`.
+ */
+export function withFileExtension(name: string, extension: string): string {
+  return name.toLowerCase().endsWith(extension.toLowerCase()) ? name : `${name}${extension}`;
+}
+
 export function extractImageKeyFromProxyUrl(url: string): string | null {
   if (!SAFE_IMAGE_PROXY_PATH.test(url)) return null;
   const filename = url.slice(IMAGE_PROXY_PREFIX.length);