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/components/questions/QuestionThread.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useRef } from "react";
+import { useState, useRef, useEffect } from "react";
 
 interface FileAttachment {
   name: string;
@@ -53,6 +53,11 @@ export function QuestionThread({
   const [sending, setSending] = useState(false);
   const [error, setError] = useState("");
   const fileInputRef = useRef<HTMLInputElement>(null);
+  const messagesEndRef = useRef<HTMLDivElement>(null);
+
+  useEffect(() => {
+    messagesEndRef.current?.scrollIntoView({ behavior: "smooth" });
+  }, [messages]);
 
   async function handleFileSelect(e: React.ChangeEvent<HTMLInputElement>) {
     const selected = Array.from(e.target.files ?? []);
@@ -82,6 +87,7 @@ export function QuestionThread({
   }
 
   async function handleSend() {
+    if (uploading) return;
     if (!text.trim() && files.length === 0) return;
     setSending(true);
     setError("");
@@ -120,7 +126,7 @@ export function QuestionThread({
               className="max-w-[88%] px-3 py-2 rounded-sm text-sm"
               style={{
                 alignSelf: isMine ? "flex-end" : "flex-start",
-                background: isMine ? "#E8E8E0" : "#F5F5F0",
+                background: isMine ? "var(--muted)" : "var(--background)",
                 border: isMine ? "none" : `2px solid #E8F0D8`,
                 borderLeft: !isMine && isNew ? "3px solid #323232" : undefined,
               }}
@@ -145,7 +151,7 @@ export function QuestionThread({
                       rel="noreferrer"
                       className="flex items-center gap-1.5 text-xs px-2 py-1 rounded-sm"
                       style={{
-                        background: "#F5F5F0",
+                        background: "var(--background)",
                         border: "1px solid #AAAAAA",
                         color: "var(--foreground)",
                         width: "fit-content",
@@ -160,6 +166,7 @@ export function QuestionThread({
             </div>
           );
         })}
+        <div ref={messagesEndRef} />
       </div>
 
       {/* Queued files preview */}
@@ -169,7 +176,7 @@ export function QuestionThread({
             <div
               key={i}
               className="flex items-center gap-1 text-xs px-2 py-1 rounded-sm"
-              style={{ background: "var(--surface)", border: "1px solid var(--border)" }}
+              style={{ background: "var(--color-surface)", border: "1px solid var(--border)" }}
             >
               📎 {f.name}
               <button
@@ -189,7 +196,7 @@ export function QuestionThread({
         className="rounded-sm p-2"
         style={{
           border: "2px solid #AAAAAA",
-          background: "var(--surface)",
+          background: "var(--color-surface)",
           opacity: questionStatus === "CLOSED" ? 0.5 : 1,
           pointerEvents: questionStatus === "CLOSED" ? "none" : "auto",
         }}
@@ -205,7 +212,7 @@ export function QuestionThread({
         />
         <div
           className="flex items-center justify-between pt-2 mt-1"
-          style={{ borderTop: "1px solid #E8E8E0" }}
+          style={{ borderTop: "1px solid var(--muted)" }}
         >
           <div className="flex items-center gap-2">
             <input
@@ -221,7 +228,7 @@ export function QuestionThread({
               onClick={() => fileInputRef.current?.click()}
               disabled={uploading}
               className="text-xs px-2 py-1 rounded-sm"
-              style={{ background: "var(--surface)", border: "1px solid #AAAAAA" }}
+              style={{ background: "var(--color-surface)", border: "1px solid #AAAAAA" }}
             >
               {uploading ? "Загрузка..." : "📎 Прикрепить"}
             </button>
@@ -245,7 +252,7 @@ export function QuestionThread({
           </button>
         </div>
         {error && (
-          <p className="text-xs mt-1" style={{ color: "#c00" }}>
+          <p className="text-xs mt-1" style={{ color: "var(--destructive)" }}>
             {error}
           </p>
         )}