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>

README.md

@@ -101,7 +101,11 @@ silently stay blank instead of erroring.
   completion % at a glance
 - Cover art: uses a manually-placed `cover`/`folder`/`thumbnail`/`thumb`/
   `poster` image if a course has one, otherwise auto-generates one via
-  `ffmpeg` from a frame of the course's first video
+  `ffmpeg` - samples a few candidate frames from the first several seconds
+  of the course's first video (where an intro title card typically lives)
+  and keeps the one that compresses to the largest JPEG, a cheap proxy for
+  "has the most going on" that favors a title card/logo over a blank
+  fade-in or a plain frame of the presenter
 - **File Management** ([/unsorted](templates/unsorted.html), its own item
   ("Files") in the bottom tab bar alongside Home/Notes/Help/Settings): a
   dedicated page for everything that touches files on disk, since it needs
@@ -181,13 +185,20 @@ silently stay blank instead of erroring.
   difference doesn't hide a real duplicate. Groups by similarity (union of
   any two courses over the threshold, transitively) rather than showing
   raw pairs; each course in a group gets a one-click Hide, or a **Delete**
-  that permanently removes it from disk (the one destructive action in the
-  whole app - confirmed with the full path before it runs). A group can
-  also be marked **"Not a duplicate"** if the match is wrong, which
-  excludes that specific pair from future scans without touching anything
-  else that happens to match one of those courses; ignored pairs are
-  listed (and reversible) under "Ignored matches," and persist through
-  backup/restore alongside hidden paths and Next Up.
+  that permanently removes it from disk (confirmed with the full path
+  before it runs). A group can also be marked **"Not a duplicate"** if the
+  match is wrong, which excludes that specific pair from future scans
+  without touching anything else that happens to match one of those
+  courses; ignored pairs are listed (and reversible) under "Ignored
+  matches," and persist through backup/restore alongside hidden paths and
+  Next Up.
+  - *Duplicate Lesson Files*: the same idea one level down - media files
+  sitting in the same folder that look like the same lesson downloaded
+  twice (e.g. a re-download that landed alongside the original instead of
+  replacing it). Compares only within a folder, at a higher match
+  threshold than Duplicate Courses, so two different lessons on a similar
+  topic don't get flagged; shares its ignore list with Duplicate Courses.
+  Delete here removes a single file, not a whole course.
   - *Clean Up Stale References*: finds entries in the hidden-paths list,
   Next Up queue, Favorites, or Recently Viewed history that point at a path
   no longer on disk - normally from renaming/moving/deleting a course
@@ -210,10 +221,15 @@ silently stay blank instead of erroring.
 - Recently Added / Recently Viewed
 - Surprise Me — dice icon in the header, random pick weighted toward
   incomplete courses
+- Grid view shows a small progress ring in the corner of each course's
+  thumbnail (a checkmark once complete) instead of a separate bar, so
+  completion reads at a glance without switching to list view
 
 **Playback & progress**
 - Video/audio player with resize, playback-speed presets, and resume-from-
   last-position
+- Auto-play next lesson when one ends, with a cancelable few-second
+  countdown - on by default, toggle it off in Settings → Video Player
 - Keyboard shortcuts on the lesson page: Space (play/pause), ←/→ (seek
   10s), ↑/↓ (volume), `,`/`.` (step playback speed), `[`/`]` (previous/next
   lesson), `F` (fullscreen), `N` (quick-capture a note) - see Help for the
@@ -226,6 +242,11 @@ silently stay blank instead of erroring.
 - Settings → "Precompute Lengths & Cover Art" walks the whole library in
   the background to populate durations and thumbnails up front, with live
   progress
+- Settings → "Regenerate Thumbnails" deletes and re-runs auto-generated
+  cover art for every course that has one (manually-placed covers are
+  never touched) - the only way to pick up an improvement to how
+  thumbnails are picked on courses whose thumbnail was already cached
+  under the old logic, since it's otherwise served from disk forever
 
 **Notes**
 - Timestamped notes per lesson, capturable via a keyboard shortcut without