Add Notes Hub, transcript search, stale nudges, study guide export, Next Up queue, activity heatmap

Six more dashboard features, all built on the per-course progress-file
scanning infrastructure from earlier sessions:

- Notes Hub (/notes): every lesson note across the library in one place,
  newest first, reusing the existing /recent/open cross-course jump route.
- Transcript search: opt-in checkbox on the Library search box, searching
  inside .srt/.vtt files. Kept cheap by doing a raw substring match as the
  filter step and only extracting a snippet for files that actually match.
- Stale course nudges ("Pick Back Up"): courses with progress that haven't
  been touched in 14+ days.
- Study guide export: compiles a course's notes into one downloadable
  markdown file, section/lesson structure preserved.
- Next Up queue: an ordered, persisted "what to tackle next" list with
  up/down reordering and a queue button on every course row.
- Activity heatmap: a 90-day contribution-style calendar in the Library
  Stats card.

_scan_library_activity() now does one walk over every course's progress
file per dashboard load; library stats, stale courses, and the heatmap all
derive from that single scan instead of three independent ones.

Also refactored the duplicated lesson-progress-key lookup (used in three
places now) into _resolve_lesson_progress_key(), and flagged a pre-existing
bug found along the way (not fixed here, kept out of scope): subtitle files
never actually attach to a Lesson object, so the video player's caption
track has never had anything to render.

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

templates/lesson_view.html

@@ -862,6 +862,7 @@
                 📚 <a href="/reset_course" style="color: inherit; text-decoration: none;">OfflineU</a>
             </div>
             <div class="footer-links">
+                <a href="/notes">📝 Notes</a>
                 <a href="/help">❓ Help</a>
                 <a href="/settings">⚙ Settings</a>
             </div>