Phase 3: Check my work, follow-ups and turn-into-deck

Check my work (third homework mode)
- One model call with its own system prompt marks the student's own
  attempt: verdict, what's right, each issue with why + fix, and the
  corrected version that keeps everything the student got right.
  Unasked-for extras are never errors. The attempt is typed/pasted
  (photos of handwriting are documented as unsupported) and stored in
  the record so Redo re-checks the same work.
- Server: mode "check" + solution on POST /api/homework (zod, 400
  server.needSolution without one) and the same NDJSON stream; new
  checkHomework() pipeline with repair-once validation.

Follow-ups
- New POST /api/homework/refine (plain JSON, RATE_LIMIT_REFINE, zod,
  auth before body): shorter / more detail / explain, plain text back,
  language preserved.
- Shorter + More detail + Original rewrite the finished answer and the
  corrected version in place (FollowUpBar); every guide step has
  Explain step N differently; failed calls say so without losing text.

Turn into a study deck
- Any result hands its content (answer, corrected version or assembled
  guide) to New deck through a one-shot sessionStorage seed with the
  options step pre-filled.

UI
- Phone step 2 gains the third card: tapping it reveals the solution
  field + submit; desktop has three cards and the solution field shows
  with the mode. Homework history labels and share/card previews cover
  the new mode.

Verified: new phase-3 suite 20/20 against a stub Ollama (check payload,
missing-solution 400, refine validation, phone + desktop check flows,
shorter follow-up with restore, guide explain, deck hand-off) plus
regression 74/74, mobile 52/52, declutter 116/116, wizard 80/80,
animation 8/8, native 35/35, phase1 38/38, phase2 43/43, i18n 25/25.
Key parity 734/734. Lint + build clean. SW freezmo-v27.

components/homework/FollowUpBar.tsx

@@ -0,0 +1,76 @@
+"use client";
+
+import { useState } from "react";
+
+import { Button } from "@/components/ui";
+import { refineHomework } from "@/lib/client-api";
+import { useI18n } from "@/lib/i18n";
+
+/**
+ * Follow-up actions for one finished piece of homework: shorter, more detail,
+ * or back to the original. Used on the make result and the corrected version
+ * of a check.
+ */
+export function FollowUpBar({
+  text,
+  context,
+  onReplace,
+  onRestore,
+  canRestore,
+}: {
+  text: string;
+  context?: string;
+  onReplace: (next: string) => void;
+  onRestore?: () => void;
+  canRestore?: boolean;
+}) {
+  const { t } = useI18n();
+  const [busy, setBusy] = useState<"shorter" | "more" | null>(null);
+  const [failed, setFailed] = useState(false);
+
+  const run = async (instruction: "shorter" | "more") => {
+    if (busy) return;
+    setBusy(instruction);
+    setFailed(false);
+    try {
+      const next = await refineHomework({ instruction, text, context });
+      onReplace(next);
+    } catch {
+      setFailed(true);
+    } finally {
+      setBusy(null);
+    }
+  };
+
+  return (
+    <div className="flex flex-wrap items-center gap-2">
+      <Button
+        size="sm"
+        variant="secondary"
+        className="touch-target"
+        loading={busy === "shorter"}
+        disabled={Boolean(busy)}
+        onClick={() => void run("shorter")}
+      >
+        {t("homework.followup.shorter")}
+      </Button>
+      <Button
+        size="sm"
+        variant="secondary"
+        className="touch-target"
+        loading={busy === "more"}
+        disabled={Boolean(busy)}
+        onClick={() => void run("more")}
+      >
+        {t("homework.followup.more")}
+      </Button>
+      {busy ? <span className="text-[12px] text-[var(--text-subtle)]">{t("homework.followup.working")}</span> : null}
+      {canRestore && onRestore ? (
+        <Button size="sm" variant="ghost" className="touch-target" disabled={Boolean(busy)} onClick={onRestore}>
+          {t("homework.followup.restore")}
+        </Button>
+      ) : null}
+      {failed ? <span className="text-[12px] font-medium text-rose-600 dark:text-rose-300">{t("homework.followup.failed")}</span> : null}
+    </div>
+  );
+}