feat: print includes answer only after unlocking solutions

- If solutions already unlocked (full mode), print button shows '打印题目+答案'
- If not unlocked, print button shows '打印题目' (problem only)
- No longer forces unlock when printing

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/app/problems/[id]/page.tsx

@@ -950,14 +950,18 @@ export default function ProblemDetailPage() {
 
   const printProblemWithAnswer = async () => {
     let resolved = solutionData;
-    if (resolved?.access?.mode !== "full") {
-      const fetched = await loadSolutions("full");
-      if (fetched) resolved = fetched;
-    } else if ((resolved?.items.length ?? 0) === 0) {
+    // If user has already unlocked solutions (full mode), use them directly
+    if (resolved?.access?.mode === "full" && (resolved?.items.length ?? 0) > 0) {
+      // Already have full solutions loaded — use as-is
+    } else if (resolved?.access?.mode === "full") {
+      // Full access but items not loaded yet — fetch
       const fetched = await loadSolutions("full");
       if (fetched) resolved = fetched;
     }
-    setPrintAnswerMarkdown(buildPrintableAnswerMarkdown(resolved, tx));
+    // If not in full mode, just print problem without answer
+    setPrintAnswerMarkdown(
+      resolved?.access?.mode === "full" ? buildPrintableAnswerMarkdown(resolved, tx) : ""
+    );
     await new Promise<void>((resolve) => {
       window.requestAnimationFrame(() => {
         window.requestAnimationFrame(() => resolve());
@@ -997,7 +1001,11 @@ export default function ProblemDetailPage() {
                 onClick={() => void printProblemWithAnswer()}
                 disabled={solutionLoading}
               >
-                {solutionLoading ? tx("卷轴准备中...", "Scribing...") : tx("打印卷轴", "Scribe Scroll")}
+                {solutionLoading
+                  ? tx("卷轴准备中...", "Scribing...")
+                  : solutionData?.access?.mode === "full"
+                    ? tx("🖨️ 打印题目+答案", "🖨️ Print with Answer")
+                    : tx("🖨️ 打印题目", "🖨️ Print Problem")}
               </button>
               <button
                 className="mc-btn text-sm py-1"