Fix student questions pages: CSS tokens, scroll, upload guard, S3 path

- Replace non-existent --surface/--surface-muted/--border-strong with actual
  design-system tokens (--color-surface, --background, --foreground, --muted)
- Remove tmp/ segment from S3 upload key in question-upload route
- Add auto-scroll to bottom on new message in QuestionThread
- Block Send while file upload is in progress (uploading guard)
- Replace <a> with Next.js <Link> in new question page back-link
- Replace hardcoded #c00 error color with var(--destructive) in both files
- Replace hardcoded #E8E8E0/#F5F5F0 hex backgrounds with CSS tokens

src/app/api/student/question-upload/route.ts

@@ -35,7 +35,7 @@ export async function POST(req: NextRequest) {
     return NextResponse.json({ error: "Недопустимое расширение файла" }, { status: 415 });
   }
 
-  const key = `questions/tmp/${session.user.id}/${randomUUID()}.${ext}`;
+  const key = `questions/${session.user.id}/${randomUUID()}.${ext}`;
   const buffer = Buffer.from(await file.arrayBuffer());
   const url = await uploadFile(key, buffer, file.type);