fix(ct-math): практика возвращала меньше count + перенос заголовков в навигации урока

1) exam-prep practice (strategy=random) возвращал около 0.6 от count: функция
   distributeByDifficulty раскладывает count по 5 уровням сложности, а у трека
   ctmath задания только уровней 1-3 (уровни 4-5 пустые) -> часть выборки терялась
   (20 -> 12, 15 -> 10, 10 -> 6). В pickRandomByDifficulty добавлен добор до count
   из доступных уровней. Трек math9 не затронут (там добор не требуется).
2) lesson.html: .lesson-nav-btn-title был inline-span, поэтому max-width и ellipsis
   игнорировались и длинные заголовки вылезали за кнопку. Добавлен display:block.

Бэкенд-правка требует перезапуска сервера; фронт-правка видна после Ctrl+F5.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/routes/exam-prep.js

@@ -630,13 +630,15 @@ function pickRandomByDifficulty(examKey, count, excludeSlugs) {
     ? `AND (subtopic IS NULL OR subtopic NOT IN (${exParams.map(() => '?').join(',')}))`
     : '';
 
+  const COLS = `id, task_idx, variant, task_type, text_html, figure_html, opts_json,
+             answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph`;
   const out = [];
+  const seen = new Set();
   for (let d = 1; d <= 5; d++) {
     const limit = dist[d - 1];
     if (limit === 0) continue;
     const sql = `
-      SELECT id, task_idx, variant, task_type, text_html, figure_html, opts_json,
-             answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph
+      SELECT ${COLS}
         FROM exam_tasks
        WHERE exam_key = ? AND task_type IN ('mc','open')
          AND difficulty = ?
@@ -646,8 +648,25 @@ function pickRandomByDifficulty(examKey, count, excludeSlugs) {
     const args = exParams
       ? [examKey, d, ...exParams, limit]
       : [examKey, d, limit];
+    for (const r of db.prepare(sql).all(...args)) { if (!seen.has(r.id)) { seen.add(r.id); out.push(r); } }
+  }
+  // Backfill to `count` from any difficulty — covers tracks whose tasks don't
+  // span all 5 difficulty levels (otherwise empty levels would shrink the batch).
+  if (out.length < count) {
+    const ids = [...seen];
+    const notIn = ids.length ? `AND id NOT IN (${ids.map(() => '?').join(',')})` : '';
+    const sql = `
+      SELECT ${COLS}
+        FROM exam_tasks
+       WHERE exam_key = ? AND task_type IN ('mc','open')
+         ${exClause}
+         ${notIn}
+       ORDER BY RANDOM()
+       LIMIT ?`;
+    const args = [examKey, ...(exParams || []), ...ids, count - out.length];
     out.push(...db.prepare(sql).all(...args));
   }
+  out.sort((a, b) => (a.difficulty || 0) - (b.difficulty || 0));
   return out;
 }