Add school name and logo to home screen and admin header

- School name now appears on the student home screen and updates the page title
- Logo can be uploaded in Settings (PNG/JPG/GIF/WebP/SVG, max 2 MB); stored in the data volume so it persists across container restarts
- When a logo is uploaded the text school name is hidden in favour of the logo
- Admin header shows the school name instead of hardcoded "Homeschool", updates live when saved
- New server endpoints: GET /api/logo, POST /api/admin/upload-logo, DELETE /api/admin/logo
- /api/public-settings now includes school_name and has_logo

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

public/index.html

@@ -34,11 +34,12 @@
 
   <!-- Kid picker -->
   <section id="view-home" class="view">
-    <h1 class="app-title">🎓 Homeschool</h1>
+    <h1 class="app-title" id="school-name-title">🎓 Homeschool</h1>
     <p class="subtitle">Who's learning today?</p>
     <div id="kid-cards" class="kid-cards"></div>
     <p id="no-kids" class="hint" hidden>No kids yet — a parent needs to add them.</p>
     <a class="parent-dash-card" href="admin.html">🔐 Parent dashboard</a>
+    <img id="school-logo" src="" alt="School logo" hidden>
   </section>
 
   <!-- Kid home: Today's work + nav -->