Add worksheet short-answer grading view, student inbox, and parent-student messaging

- Grading queue now shows short-answer questions and student responses inline so the parent can read answers before scoring
- Student inbox tab shows all graded items with score, percentage, and parent feedback; unread badge appears on login
- Parent-student messaging: students send messages from the Inbox tab; Messages panel in admin shows threads with unread count badge, full conversation history, and reply compose

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

public/admin.html

@@ -28,6 +28,7 @@
         <button class="nav-pill" data-panel="courses">📚 Courses</button>
         <button class="nav-pill" data-panel="planner">🗓️ Planner</button>
         <button class="nav-pill" data-panel="grading">✅ Grading</button>
+        <button class="nav-pill" data-panel="messages">💬 Messages</button>
         <button class="nav-pill" data-panel="gradebook">📊 Gradebook</button>
         <button class="nav-pill" data-panel="spelling">✏️ Spelling</button>
         <button class="nav-pill" data-panel="decks">🗂️ Flashcards</button>
@@ -376,6 +377,17 @@
         </div>
       </section>
 
+      <!-- Messages -->
+      <section id="panel-messages" class="panel">
+        <h1>Messages</h1>
+        <div class="msg-admin-cols">
+          <div id="msg-thread-list" class="msg-thread-list"></div>
+          <div id="msg-admin-thread" class="msg-admin-thread">
+            <p class="hint" style="padding:1rem">Select a student to view the conversation.</p>
+          </div>
+        </div>
+      </section>
+
       <!-- Gradebook -->
       <section id="panel-gradebook" class="panel">
         <h1>Gradebook</h1>