Report question: skip a broken card and tell the server

A generated matching card arrived on a phone with three identical
definitions (the generation guards now drop those), but there was no way
to get past a bad card from inside a session. The feed chrome (deck
study, review, mistakes and exam) now has the red "!" control: one tap
records the question, skips to the next card and shows an Undo toast.

- lib/question-reports.ts: the local list (freezmo.reports.v1, zod on
  read, capped 200) is the skip state — reported cards leave every
  session type, count as "not attempted" in the results instead of
  wrong, and Undo takes a report back. A deck where everything is
  reported stays studiable so it can be regenerated.
- POST /api/reports appends one JSON line per report to
  REPORTS_DIR/reports.jsonl (shared/freezmo/reports in production, the
  dev data dir in dev). Deliberately write-only: the question text is
  readable on the server (that is the point — prompt tuning) but never
  over HTTP, since the app has no auth. Best effort: offline reports stay
  local.
- The button: a red outlined rounded square with "!", the chrome's
  control size, in both feed headers (hidden on the results slide).
- config: REPORTS_DIR + RATE_LIMIT_REPORTS (20); the deploy creates the
  shared reports dir; the dev unit and AGENTS.md note the new server
  data location.
- Suites: phase-1 (append + validation + no GET), phase-2 (button, toast,
  server POST, local copy, skip counter, hidden next session, Undo, exam
  pool). SW -> freezmo-v38.

components/exam/ExamScreen.tsx

@@ -4,6 +4,7 @@ import { useMemo, useState, useSyncExternalStore } from "react";
 import { useRouter } from "next/navigation";
 
 import { SessionFeed, type SessionItem } from "@/components/SessionFeed";
+import { isReported } from "@/lib/question-reports";
 import { Button } from "@/components/ui";
 import { haptic } from "@/lib/haptics";
 import { recordAnswer } from "@/lib/history";
@@ -33,7 +34,9 @@ export function ExamScreen() {
     () =>
       decks
         .filter((deck) => selected.has(deck.id))
-        .flatMap((deck) => deck.activities.map((activity) => ({ deck, activity }))),
+        .flatMap((deck) => deck.activities.map((activity) => ({ deck, activity })))
+        // Reported questions are out of rotation (see lib/question-reports).
+        .filter(({ deck, activity }) => !isReported(deck.id, activity.id)),
     [decks, selected],
   );
   const effectiveCount = Math.min(count, pool.length);