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/api/src/routes/stems.ts

@@ -5,13 +5,12 @@ import { requestStemUploadUrlSchema, createStemSchema } from '@music-hub/shared'
 import { tracks, stems, projectMembers } from '@music-hub/db';
 import { requireAuth } from '../middleware/auth.js';
 import { createUploadUrl, getObjectBuffer, deleteObject } from '../storage/s3.js';
-import { zipSync } from 'fflate';
+import { zip } from 'fflate';
 import type { AppEnv } from '../types.js';
 
 export const stemRoutes = new Hono<AppEnv>()
   .use('*', requireAuth)
 
-  // List stems for a track
   .get('/track/:trackId', async (c) => {
     const db = c.get('db');
     const userId = c.get('userId');
@@ -36,7 +35,6 @@ export const stemRoutes = new Hono<AppEnv>()
     return c.json({ stems: trackStems });
   })
 
-  // Request presigned upload URL
   .post('/track/:trackId/upload-url', zValidator('json', requestStemUploadUrlSchema), async (c) => {
     const db = c.get('db');
     const userId = c.get('userId');
@@ -60,7 +58,6 @@ export const stemRoutes = new Hono<AppEnv>()
     return c.json({ uploadUrl, fileKey, stemId });
   })
 
-  // Register stem after upload
   .post('/track/:trackId', zValidator('json', createStemSchema), async (c) => {
     const db = c.get('db');
     const userId = c.get('userId');
@@ -93,7 +90,6 @@ export const stemRoutes = new Hono<AppEnv>()
     return c.json({ stem }, 201);
   })
 
-  // Delete stem
   .delete('/:id', async (c) => {
     const db = c.get('db');
     const userId = c.get('userId');
@@ -103,10 +99,12 @@ export const stemRoutes = new Hono<AppEnv>()
     if (!stem) return c.json({ error: 'Not found' }, 404);
 
     const [track] = await db.select().from(tracks).where(eq(tracks.id, stem.trackId)).limit(1);
+    if (!track) return c.json({ error: 'Not found' }, 404);
+
     const [membership] = await db
       .select()
       .from(projectMembers)
-      .where(and(eq(projectMembers.projectId, track!.projectId), eq(projectMembers.userId, userId)))
+      .where(and(eq(projectMembers.projectId, track.projectId), eq(projectMembers.userId, userId)))
       .limit(1);
 
     if (!membership || (membership.role !== 'owner' && stem.createdById !== userId)) {
@@ -118,7 +116,6 @@ export const stemRoutes = new Hono<AppEnv>()
     return c.json({ message: 'Stem deleted' });
   })
 
-  // Download all stems as ZIP
   .get('/track/:trackId/download-zip', async (c) => {
     const db = c.get('db');
     const userId = c.get('userId');
@@ -142,23 +139,21 @@ export const stemRoutes = new Hono<AppEnv>()
 
     if (trackStems.length === 0) return c.json({ error: 'No stems found' }, 404);
 
-    // Download all files and build ZIP
-    const files: Record<string, Uint8Array> = {};
-    await Promise.all(
-      trackStems.map(async (stem) => {
-        const buf = await getObjectBuffer(stem.fileKey);
-        files[stem.originalFileName] = buf;
-      }),
+    const fileEntries = await Promise.all(
+      trackStems.map(async (stem) => [stem.originalFileName, await getObjectBuffer(stem.fileKey)] as const),
     );
+    const files = Object.fromEntries(fileEntries) as Record<string, Uint8Array>;
 
-    const zipped = zipSync(files);
+    const zipped = await new Promise<Uint8Array>((resolve, reject) => {
+      zip(files, (err, data) => (err ? reject(err) : resolve(data)));
+    });
     const zipName = `${track.name.replace(/[^a-z0-9]/gi, '-').toLowerCase()}-stems.zip`;
+    const body = zipped.buffer.slice(zipped.byteOffset, zipped.byteOffset + zipped.byteLength) as ArrayBuffer;
 
-    return new Response(zipped, {
+    return new Response(new Blob([body], { type: 'application/zip' }), {
       headers: {
-        'Content-Type': 'application/zip',
         'Content-Disposition': `attachment; filename="${zipName}"`,
-        'Content-Length': String(zipped.length),
+        'Content-Length': String(zipped.byteLength),
       },
     });
   });