Gradebook unit cards, scroll/state preservation on course edit

- Gradebook now groups items by unit with collapsible cards (same card
  style as course detail) and a Course overall summary table at the bottom
- server.js: add unit_id/unit_name to gradebook API so items can be grouped
- Course detail: preserve which units are open and scroll position across
  any in-place re-render (add unit, delete unit, save meta, return from
  item editor) — no more jumping to the top with everything reset

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server.js

@@ -884,7 +884,8 @@ app.get('/api/gradebook/:courseId', requirePin, (req, res) => {
   if (!course) return res.status(404).json({ error: 'No such course' });
 
   const gradableItems = db.prepare(`
-    SELECT i.id, i.title, i.type, i.points, i.due_date, i.retake_policy
+    SELECT i.id, i.title, i.type, i.points, i.due_date, i.retake_policy,
+           u.id AS unit_id, u.name AS unit_name, u.sort AS unit_sort
     FROM items i JOIN units u ON u.id = i.unit_id
     WHERE u.course_id = ? AND i.type IN (${GRADABLE_TYPES.map(() => '?').join(',')})
     ORDER BY u.sort, i.sort, i.id