Redesign expandable lists to a leaner, flat style

Course tree (lesson-page sidebar + dashboard's loaded-course view) and
File Management's Manage Library browser both move from boxed,
bordered rows to a flat list: no per-row background/border, a small
leading chevron that doubles as the expand affordance instead of a
separate right-edge button, indentation for hierarchy. The chevron's
rotation is pure CSS (:has(+ .tree-content.expanded) / :has(+
.curate-children.expanded)) rather than JS swapping glyph text, which
also let a fair amount of now-redundant JS come out.

The course tree's new styles are scoped under .tree-container (course
dashboard) and .lesson-sidebar (lesson page) so they don't leak into
the Library folder browser and other rows that reuse the same base
.tree-header/.lesson-item/etc. class names elsewhere in
course_dashboard.html - confirmed those are visually untouched.

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

README.md

@@ -169,7 +169,8 @@ silently stay blank instead of erroring.
   rather than a level to browse into); every row also has a checkbox, so
   several items - found via search or expanded across different tree
   levels - can be hidden, shown, or moved to the same destination together
-  in one batch instead of one at a time.
+  in one batch instead of one at a time. Rows use the same flat, leading-
+  chevron styling as the course tree (see Playback & progress below).
   - *Undo*: a "Last action: ... [Undo]" bar appears after any move or
   rename (Sort Unsorted apply, Bulk Rename apply, a Manage Library
   move/rename, a bulk move) and reverses the whole batch in one click.
@@ -242,7 +243,11 @@ silently stay blank instead of erroring.
   "Expand all" link (toggling to "Collapse all") opens or closes every
   section at once - same control on the dashboard's course tree.
   Shares its rendering with the loaded-course dashboard view (`templates/
-  _course_tree.html`), so the two always look and behave the same.
+  _course_tree.html`), so the two always look and behave the same. Flat
+  list styling - no per-row box or border, a small leading chevron that
+  doubles as the expand affordance instead of a separate right-edge
+  button, indentation for hierarchy - for a denser, cleaner look than
+  boxed rows would give at this list length.
 - 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