Redesign student UI and add OnlyOffice document integration

- Student section: new hero gradient header, fixed bottom nav bar with
  icon+label pills (Today/Courses/Spelling/Inbox/Docs)
- Today rows: card style with left accent stripe, lift on hover
- Course cards: grid layout with top color bar per course
- Docs tab: list view with doc cards, OnlyOffice editor embedded via
  iframe using server-signed JWT config
- Server: documents table, CRUD routes, OnlyOffice callback endpoint
  for auto-save, /api/docs/:id/editor-config returns signed config
- Blank DOCX template embedded as base64 for new document creation
- OnlyOffice server: https://onlyoffice.necloud.us (set ONLYOFFICE_JWT_SECRET
  env var to match your Document Server JWT secret)

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

public/admin.js

@@ -216,7 +216,7 @@ async function loadDashboard() {
 
     const scores = recentScores.scores || recentScores || [];
     scoresEl.innerHTML = scores.length ? scores.slice(0, 8).map(sc => {
-      const pct = sc.total > 0 ? Math.round(sc.score / sc.total * 100) : null;
+      const pct = sc.points_possible > 0 ? Math.round(sc.score / sc.points_possible * 100) : null;
       const cls = pct === null ? '' : pct >= 80 ? '' : pct >= 60 ? ' mid' : ' low';
       const kidName = students.find(s => s.id === sc.student_id)?.name || '';
       return `<div class="dash-score-item">