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/admin.html

@@ -357,6 +357,9 @@
       <!-- Grading queue -->
       <section id="panel-grading" class="panel">
         <h1>Grading Queue</h1>
+        <div class="inline-form" style="margin-bottom:1rem">
+          <label>Student <select id="grading-student-filter"><option value="">All students</option></select></label>
+        </div>
         <div id="grading-rows"></div>
         <p id="grading-empty" class="hint" hidden>Nothing waiting to be graded 🎉</p>
         <!-- Evidence lightbox -->