Add spelling test polish: date filter, CSV export, student history, planner badges

- Gradebook: period dropdown (all time / 30 / 90 days) and CSV export for
  Spelling Test History section
- Student portal spelling tab: shows last 10 tests as a Recent Tests list
- Planner: completed standalone spelling tests appear as read-only ⭐ badges
  on the day they were taken (week and month views)
- Removed dead s.tests query from /api/overview (now served by dedicated endpoints)
- New endpoints: GET /api/spelling-tests/csv, GET /api/students/:id/tests,
  GET /api/admin/students/:id/tests-range

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

public/index.html

@@ -55,6 +55,7 @@
         <button id="btn-test" class="big-btn test">⭐ Friday Test</button>
         <button id="btn-print-list" class="big-btn print-list">🖨 Print my words</button>
       </div>
+      <div id="kid-test-history"></div>
     </div>
   </section>