Add autoplay, progress rings, duplicate-lesson detection, title-card thumbnails

- Auto-play next lesson on end, with a cancelable countdown and a
  Settings toggle (default on).
- Grid-view course cards show a small progress ring (checkmark at
  100%) instead of a separate bar.
- Duplicate Lesson Files: scans within each course/folder for media
  files that look like the same lesson downloaded twice, with per-file
  delete and a shared ignore list with Duplicate Courses.
- Auto-generated cover art now samples a few early candidate frames
  and keeps the largest JPEG, favoring an intro title card over a
  blank fade-in or a plain presenter frame.
- Settings -> "Regenerate Thumbnails" re-runs that logic for every
  course with an auto-generated thumbnail (never touches manual
  covers), so already-cached thumbnails can pick up the improvement.
- Add .gitignore for __pycache__/.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/lesson_view.html

@@ -164,6 +164,17 @@
             color: var(--text-muted);
             margin-top: 6px;
         }
+        .autoplay-banner {
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            gap: 12px;
+            margin-top: 10px;
+            padding: 10px 14px;
+            background: var(--bg-tertiary);
+            border-radius: var(--radius);
+            font-size: 0.9em;
+        }
         .speed-controls {
             display: flex;
             align-items: center;
@@ -539,6 +550,13 @@
             <p class="resize-hint">↘ Drag the bottom-right corner to resize the player</p>
             {% endif %}
 
+            {% if next_lesson %}
+            <div class="autoplay-banner" id="autoplay-banner" style="display: none;">
+                <span>Playing next lesson in <span id="autoplay-countdown">5</span>s…</span>
+                <button type="button" class="btn btn-secondary" onclick="cancelAutoplay()">Cancel</button>
+            </div>
+            {% endif %}
+
             {% if lesson.audio_file %}
             <h3>Audio</h3>
             <audio controls preload="metadata" id="audio-player">
@@ -800,13 +818,16 @@
             // Restore the persisted playback speed and highlight the active
             // button; clicking a button applies it immediately and persists
             // it via /api/settings, mirroring the video-size pattern above.
+            let autoplayNextEnabled = true;
             if (activeMedia) {
                 fetch('/api/settings')
                     .then(r => r.json())
                     .then(data => {
-                        const speed = (data.settings || {}).playback_speed || '1';
+                        const settings = data.settings || {};
+                        const speed = settings.playback_speed || '1';
                         activeMedia.playbackRate = parseFloat(speed);
                         highlightSpeedButton(speed);
+                        autoplayNextEnabled = settings.autoplay_next !== false;
                     })
                     .catch(() => {});
             }
@@ -827,6 +848,8 @@
                 }).catch(() => {});
             }
 
+            const NEXT_LESSON_URL = {{ ('/lesson/' ~ next_lesson)|tojson if next_lesson else 'null' }};
+
             // ---- Outline topic chooser + push-on-leave ----
             const LESSON_PATH = {{ lesson_path|tojson }};
             const LESSON_TITLE = {{ lesson.title|tojson }};
@@ -1210,7 +1233,43 @@
                 activeMedia.addEventListener('ended', function() {
                     saveProgress(activeMedia.currentTime, true);
                     markAsCompleted();
+                    if (NEXT_LESSON_URL && autoplayNextEnabled) startAutoplayCountdown();
                 });
+
+                // Replaying the just-ended lesson (rather than letting it sit)
+                // is a clear enough signal the user doesn't want to leave yet.
+                activeMedia.addEventListener('play', cancelAutoplay);
+            }
+
+            let autoplayTimer = null;
+            let autoplaySecondsLeft = 5;
+
+            function startAutoplayCountdown() {
+                const banner = document.getElementById('autoplay-banner');
+                const countdownEl = document.getElementById('autoplay-countdown');
+                if (!banner || !countdownEl) return;
+                autoplaySecondsLeft = 5;
+                countdownEl.textContent = autoplaySecondsLeft;
+                banner.style.display = 'flex';
+                autoplayTimer = setInterval(function() {
+                    autoplaySecondsLeft -= 1;
+                    if (autoplaySecondsLeft <= 0) {
+                        clearInterval(autoplayTimer);
+                        autoplayTimer = null;
+                        window.location.href = NEXT_LESSON_URL;
+                        return;
+                    }
+                    countdownEl.textContent = autoplaySecondsLeft;
+                }, 1000);
+            }
+
+            function cancelAutoplay() {
+                if (autoplayTimer) {
+                    clearInterval(autoplayTimer);
+                    autoplayTimer = null;
+                }
+                const banner = document.getElementById('autoplay-banner');
+                if (banner) banner.style.display = 'none';
             }
 
             function saveProgress(progressSeconds, completed = false) {