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/(student)/questions/new/page.tsx

@@ -2,6 +2,7 @@
 
 import { useState } from "react";
 import { useRouter } from "next/navigation";
+import Link from "next/link";
 
 export default function NewQuestionPage() {
   const router = useRouter();
@@ -38,13 +39,13 @@ export default function NewQuestionPage() {
   return (
     <div className="max-w-xl mx-auto px-4 py-8">
       <div className="flex items-center gap-3 mb-6">
-        <a
+        <Link
           href="/questions"
           className="text-sm"
           style={{ color: "var(--muted-foreground)", textDecoration: "underline" }}
         >
           ← Все вопросы
-        </a>
+        </Link>
       </div>
 
       <h1 className="text-xl font-bold mb-6" style={{ color: "var(--foreground)" }}>
@@ -68,7 +69,7 @@ export default function NewQuestionPage() {
             className="w-full text-sm px-3 py-2 outline-none"
             style={{
               border: "2px solid var(--border)",
-              background: "var(--surface)",
+              background: "var(--color-surface)",
               color: "var(--foreground)",
             }}
           />
@@ -90,14 +91,14 @@ export default function NewQuestionPage() {
             className="w-full text-sm px-3 py-2 outline-none resize-none"
             style={{
               border: "2px solid var(--border)",
-              background: "var(--surface)",
+              background: "var(--color-surface)",
               color: "var(--foreground)",
             }}
           />
         </div>
 
         {error && (
-          <p className="text-sm" style={{ color: "#c00" }}>
+          <p className="text-sm" style={{ color: "var(--destructive)" }}>
             {error}
           </p>
         )}