Add auto-thumbnails, library-wide remaining time, backup restore

Auto-generated course thumbnails: find_course_thumbnail() falls back to
grabbing a frame from a course's first video via ffmpeg when there's no
manual cover image, cached to .offlineu_thumbnail.jpg so it only ever
runs once. Folded into the "Precompute" prewarm button, now "Precompute
Lengths & Cover Art".

Library-wide "Remaining" stat: sums every course's already-cached
duration (no ffprobe, just a JSON read) into the dashboard's Library
Stats card, alongside the existing "Watched" figure.

Backup restore: new /api/backup/restore endpoint and a "Choose Backup
File..." control in Settings, with zip-slip and missing-course guards.
Also fixed a gap in the export itself - next_up.json and
outline_config.json weren't being backed up before, so restore wouldn't
have been a true round trip.

Move Surprise Me from a full-width button above the course list to a
dice-icon button in the dashboard header, swapping with the course-name
badge depending on whether a course is loaded.

Update README to cover all of the above.

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

README.md

@@ -85,17 +85,21 @@ silently stay blank instead of erroring.
   opt-in transcript search across subtitle files
 - Course cards show media file count, total video/audio runtime, and
   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
 - Hide courses/folders from the browser without touching anything on disk;
   bulk-select to hide or queue several at once
 - Bulk rename across course/folder names (find & replace)
 
 **Dashboard**
-- Library-wide stats (courses, lessons completed, time watched, day streak)
-  and a 90-day activity heatmap
+- Library-wide stats (courses, lessons completed, time watched, time
+  remaining, day streak) and a 90-day activity heatmap
 - Next Up queue (manually curated, reorderable)
 - Pick Back Up (courses with progress that have gone stale)
 - Recently Added / Recently Viewed
-- Surprise Me — random pick, weighted toward incomplete courses
+- Surprise Me — dice icon in the header, random pick weighted toward
+  incomplete courses
 
 **Playback & progress**
 - Video/audio player with resize, playback-speed presets, and resume-from-
@@ -103,9 +107,11 @@ silently stay blank instead of erroring.
 - Auto-tracks watch progress and completion per lesson
 - Video/audio durations read via `ffprobe` and cached persistently per
   course (`.offlineu_duration_cache.json`), so total runtime and per-lesson
-  length show up before you've ever pressed play — not just after
-- Settings → "Precompute Video Lengths" walks the whole library in the
-  background to populate that cache up front, with live progress
+  length show up before you've ever pressed play — not just after; the
+  same cache backs the library-wide "time remaining" stat
+- Settings → "Precompute Lengths & Cover Art" walks the whole library in
+  the background to populate durations and thumbnails up front, with live
+  progress
 
 **Notes**
 - Timestamped notes per lesson, capturable via a keyboard shortcut without
@@ -120,9 +126,12 @@ silently stay blank instead of erroring.
   5-minute filesystem-scan cache)
 
 **Backup & integrations**
-- One-click backup export: settings, hidden-path choices, recent-view
-  history, and every course's progress/notes as a zip (not the course files
-  themselves)
+- One-click backup export and restore: settings, hidden-path choices, the
+  Next Up queue, recent-view history, Outline config, and every course's
+  progress/notes as a zip (not the course files themselves). Restore
+  overwrites current data and needs a matching course folder to already
+  exist for each course's progress to land - it's a "put my data back"
+  action, not a merge
 - Optional [Outline](https://www.getoutline.com/) integration — push a
   lesson's notes to an Outline document/collection
 
@@ -142,10 +151,11 @@ Everything under `OFFLINEU_DATA_DIR` (app-wide, not tied to a course):
 
 Per-course, written inside the course's own folder on the library volume:
 
-| File                              | Contents                                          |
-| ----------------------------------- | ---------------------------------------------------- |
-| `.offlineu_progress.json`           | Per-lesson completed/progress/duration/notes         |
-| `.offlineu_duration_cache.json`     | ffprobe duration cache, keyed by (path, size, mtime) |
+| File                              | Contents                                              |
+| ----------------------------------- | -------------------------------------------------------- |
+| `.offlineu_progress.json`           | Per-lesson completed/progress/duration/notes             |
+| `.offlineu_duration_cache.json`     | ffprobe duration cache, keyed by (path, size, mtime)     |
+| `.offlineu_thumbnail.jpg`           | Auto-generated cover art (only if no manual cover exists) |
 
 ---
 
@@ -162,7 +172,8 @@ MyCourse/
 │   └── resources/
 │       └── extras.md
 ├── .offlineu_progress.json        ← created automatically
-└── .offlineu_duration_cache.json  ← created automatically
+├── .offlineu_duration_cache.json  ← created automatically
+└── .offlineu_thumbnail.jpg        ← created automatically, only if no manual cover exists
 ```
 
 No metadata files needed — course/section/lesson names come straight from