Add kid engagement features and planner drag-and-drop

- Streak tracker: server-side streak counting per kid, celebration overlay
  when all today's tasks are done for the first time that day
- Kid themes: 5 colour palettes (blue/green/purple/orange/pink), picked via
  colour dots on the kid home screen; parents can set it from the Kids panel
- Grading badge: Grading nav pill turns red and shows a pending count
- Print worksheets: 🖨 Worksheet button per word list in admin Spelling panel;
  kids also get "Print my words" on the Spelling tab
- Parent feedback: gold 💬 badge on Today rows with unread comments; more
  prominent callout styling inside the assignment view
- Planner drag-and-drop: tasks can be dragged between days; PATCH
  /api/schedule/:id moves the task, drop zones highlight on hover

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

db.js

@@ -224,6 +224,10 @@ tryAlter(`ALTER TABLE schedule ADD COLUMN status TEXT NOT NULL DEFAULT 'not_star
 tryAlter(`ALTER TABLE schedule ADD COLUMN evidence_notes TEXT`);
 tryAlter(`ALTER TABLE words ADD COLUMN definition TEXT NOT NULL DEFAULT ''`);
 tryAlter(`ALTER TABLE schedule ADD COLUMN evidence_photo TEXT`);
+tryAlter(`ALTER TABLE students ADD COLUMN theme TEXT NOT NULL DEFAULT 'blue'`);
+tryAlter(`ALTER TABLE students ADD COLUMN streak_date TEXT`);
+tryAlter(`ALTER TABLE students ADD COLUMN streak_count INTEGER NOT NULL DEFAULT 0`);
+tryAlter(`ALTER TABLE students ADD COLUMN best_streak INTEGER NOT NULL DEFAULT 0`);
 
 export const sha256 = (s) => createHash('sha256').update(String(s)).digest('hex');