refactor: simplify stem code per review

- async zip() instead of zipSync (non-blocking for large files)
- null check for track in DELETE endpoint (was non-null assertion)
- formatFileSize extracted to format.ts, imported in StemList
- Stem type exported from StemList, removed duplicate in +page.svelte
- files[idx].progress = p direct Svelte 5 mutation (no spread)
- remove narrative comments from stems.ts

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

apps/web/src/lib/components/audio/StemUploadDropzone.svelte

@@ -72,7 +72,7 @@
 
       step = 'S3';
       await uploadWithProgress(uploadUrl, file, mimeType, (p) => {
-        files[idx] = { ...files[idx], progress: p };
+        files[idx].progress = p;
       });
 
       step = 'DB';