Fix 7 UX pain points: grading filter, next-item nav, lesson prose, gradebook calc, auto-schedule skip, lesson item defaults, planner improvements

- Grading: add student filter dropdown, Enter-to-save, and point max always defaults to 100
- Student side: show next-item/back-to-course actions after completing a lesson or assignment
- Lesson body: apply prose typography styles (headings, lists, blockquotes, links, hr)
- Gradebook: fix % display when points_possible is 0 (default to 100 on grade save)
- Auto-schedule: skip items that already have a submission
- Item editor: default new/existing item point values to 100 instead of 10
- Planner: friendly week date header, per-day short date, today highlight, course color on task pills, done toggle, within-day drag reorder, Today button

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

public/index.html

@@ -101,6 +101,7 @@
       <h1 id="lesson-title"></h1>
       <div id="lesson-body" class="lesson-body"></div>
       <button id="lesson-done-btn" class="check-btn">Done reading ✓</button>
+      <div id="lesson-next-actions" class="next-actions" hidden></div>
     </div>
   </section>
 
@@ -135,6 +136,7 @@
       <button id="assignment-done-btn" class="check-btn">Mark as done ✓</button>
       <div id="assignment-status" class="status-banner" hidden></div>
       <div id="assignment-comment" class="parent-comment" hidden></div>
+      <div id="assignment-next-actions" class="next-actions" hidden></div>
     </div>
   </section>