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/styles.css

@@ -53,6 +53,8 @@ body {
 }
 
 .app-title { font-size: 2.4rem; margin-bottom: 0.5rem; }
+
+#school-logo { display: block; margin: 2.5rem auto 0; max-width: min(520px, 92vw); height: auto; }
 .subtitle { font-size: 1.25rem; color: var(--muted); margin-bottom: 2rem; }
 .hint { color: var(--muted); margin: 1rem 0; }
 .error { color: var(--bad); margin: 0.5rem 0; }