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>

package.json

@@ -11,7 +11,8 @@
   "author": "NateEmmert",
   "license": "ISC",
   "dependencies": {
-    "express": "^5.1.0"
+    "express": "^5.1.0",
+    "mammoth": "^1.12.0"
   },
   "engines": {
     "node": ">=24"