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/settings.html

@@ -359,6 +359,12 @@
                     <span id="prewarm-durations-status" style="font-size: 0.85em; color: var(--text-muted);"></span>
                 </div>
                 <span class="setting-desc">Reads every video/audio file's length and generates cover art for any course without one, up front - so course cards in the Library show their runtime and artwork immediately instead of computing it the first time each course is viewed.</span>
+
+                <div style="display: flex; align-items: center; gap: 10px; margin-top: 10px;">
+                    <button class="btn btn-secondary btn-sm" id="regen-thumbnails-btn" onclick="startThumbnailRegen()">{{ icons.icon('refresh', 14) }} Regenerate Thumbnails</button>
+                    <span id="regen-thumbnails-status" style="font-size: 0.85em; color: var(--text-muted);"></span>
+                </div>
+                <span class="setting-desc">Re-runs auto-generated cover art from scratch for every course that has one (not manually-placed covers) - useful after an improvement to how thumbnails are picked, since existing ones are otherwise cached forever.</span>
             </div>
         </div>
 
@@ -412,6 +418,12 @@
                     {% endfor %}
                 </select>
             </div>
+            <div class="setting-row">
+                <label for="autoplay_next">Auto-play next lesson
+                    <span class="setting-desc">When a lesson finishes, automatically move on to the next one after a few seconds (cancelable)</span>
+                </label>
+                <input type="checkbox" id="autoplay_next" data-setting="autoplay_next" {% if settings.autoplay_next %}checked{% endif %}>
+            </div>
         </div>
 
         <div class="card">
@@ -541,6 +553,10 @@
             el.addEventListener('change', () => saveSetting(el.dataset.setting, el.value));
         });
 
+        document.querySelectorAll('input[type="checkbox"][data-setting]').forEach(el => {
+            el.addEventListener('change', () => saveSetting(el.dataset.setting, el.checked));
+        });
+
         const accentPicker = document.getElementById('accent-picker');
         const accentHex = document.getElementById('accent-hex');
 
@@ -648,6 +664,70 @@
                 .catch(() => {});
         })();
 
+        function updateThumbnailRegenStatus(data) {
+            const btn = document.getElementById('regen-thumbnails-btn');
+            const status = document.getElementById('regen-thumbnails-status');
+            if (data.running) {
+                btn.disabled = true;
+                status.style.color = 'var(--text-muted)';
+                status.textContent = `Regenerating… ${data.done}/${data.total}`;
+                return;
+            }
+            btn.disabled = false;
+            if (data.error) {
+                status.style.color = 'var(--error)';
+                status.textContent = `Error: ${data.error}`;
+            } else if (data.total) {
+                status.style.color = 'var(--success)';
+                status.innerHTML = `${ICON_SVGS.check} Regenerated ${data.total} thumbnail${data.total === 1 ? '' : 's'}`;
+                setTimeout(() => { status.textContent = ''; }, 5000);
+            } else {
+                status.style.color = 'var(--text-muted)';
+                status.textContent = 'No auto-generated thumbnails to regenerate';
+                setTimeout(() => { status.textContent = ''; }, 5000);
+            }
+        }
+
+        function pollThumbnailRegenStatus() {
+            fetch('/api/library/regenerate-thumbnails/status')
+                .then(r => r.json())
+                .then(data => {
+                    updateThumbnailRegenStatus(data);
+                    if (data.running) setTimeout(pollThumbnailRegenStatus, 1500);
+                })
+                .catch(() => {});
+        }
+
+        function startThumbnailRegen() {
+            const status = document.getElementById('regen-thumbnails-status');
+            document.getElementById('regen-thumbnails-btn').disabled = true;
+            status.style.color = 'var(--text-muted)';
+            status.textContent = 'Starting…';
+            fetch('/api/library/regenerate-thumbnails', { method: 'POST' })
+                .then(r => r.json())
+                .then(data => {
+                    updateThumbnailRegenStatus(data);
+                    if (data.running) setTimeout(pollThumbnailRegenStatus, 1500);
+                })
+                .catch(() => {
+                    status.style.color = 'var(--error)';
+                    status.textContent = 'Could not reach the server';
+                    document.getElementById('regen-thumbnails-btn').disabled = false;
+                });
+        }
+
+        (function resumeThumbnailRegenStatusIfRunning() {
+            fetch('/api/library/regenerate-thumbnails/status')
+                .then(r => r.json())
+                .then(data => {
+                    if (data.running) {
+                        updateThumbnailRegenStatus(data);
+                        setTimeout(pollThumbnailRegenStatus, 1500);
+                    }
+                })
+                .catch(() => {});
+        })();
+
         function handleRestoreFileChosen(input) {
             const file = input.files && input.files[0];
             input.value = '';  // allow re-choosing the same file later