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/course_dashboard.html

@@ -782,6 +782,7 @@
             transform: translateY(-2px);
         }
         .grid-card-thumb-wrap {
+            position: relative;
             width: 100%;
             aspect-ratio: 1 / 1;
             border-radius: var(--radius);
@@ -822,15 +823,32 @@
             font-size: 0.75em;
             color: var(--text-muted);
         }
-        .grid-card-progress-track {
-            height: 3px;
-            border-radius: 2px;
-            background: rgba(255, 255, 255, 0.08);
-            overflow: hidden;
+        .grid-card-progress-ring {
+            position: absolute;
+            bottom: 6px;
+            right: 6px;
+            width: 26px;
+            height: 26px;
+            border-radius: 50%;
+            background: conic-gradient(var(--accent) calc(var(--pct) * 1%), rgba(0, 0, 0, 0.4) 0);
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            box-shadow: 0 1px 3px rgba(0, 0, 0, 0.4);
         }
-        .grid-card-progress-fill {
-            height: 100%;
-            background: var(--accent);
+        .grid-card-progress-ring::before {
+            content: '';
+            position: absolute;
+            inset: 3px;
+            border-radius: 50%;
+            background: var(--bg-secondary);
+        }
+        .grid-card-progress-ring-check {
+            position: relative;
+            color: var(--accent);
+            font-size: 12px;
+            font-weight: 700;
+            line-height: 1;
         }
 
         .transcript-search-toggle {
@@ -1704,19 +1722,17 @@
                 : courseInitialHtml(item.name);
             const clickHandler = selectionMode ? `toggleSelection('${safePath}')` : `loadCoursePath('${safePath}')`;
             const pct = item.completion_percentage;
-            const progressBar = pct
-                ? `<div class="grid-card-progress-track"><div class="grid-card-progress-fill" style="width: ${pct}%;"></div></div>`
+            const ringHtml = pct
+                ? `<div class="grid-card-progress-ring" style="--pct: ${pct};" title="${pct}% complete">${pct >= 100 ? '<span class="grid-card-progress-ring-check">✓</span>' : ''}</div>`
                 : '';
             const metaBits = [`${item.media_files} media file${item.media_files === 1 ? '' : 's'}`];
             if (item.duration_display) metaBits.push(item.duration_display);
-            if (pct) metaBits.push(`${pct}% done`);
             return `
                 <div class="library-grid-card" onclick="${clickHandler}">
                     ${selectableAttrs(item)}
-                    <div class="grid-card-thumb-wrap">${iconHtml}</div>
+                    <div class="grid-card-thumb-wrap">${iconHtml}${ringHtml}</div>
                     <div class="grid-card-name" title="${escapeAttr(item.name)}">${item.name}</div>
                     <div class="grid-card-meta">${metaBits.join(' · ')}</div>
-                    ${progressBar}
                 </div>
             `;
         }