Simplify school types and persist selected schools

- Add runtime normalization of cryptic school type codes to user-friendly names
  (e.g., AC/ACC/ACCS -> "Academy", CY/CYS -> "Community")
- Update SCHOOL_TYPE_MAP in schemas.py with consolidated mappings
- Add normalize_school_type() and get_school_type_codes_for_filter() helpers
- Persist selected schools in localStorage across page refreshes
- Move "Add to Compare" button from modal footer to header

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/styles.css

@@ -1088,6 +1088,10 @@ body {
     margin-bottom: 0.25rem;
 }
 
+.modal-header .btn {
+    margin-top: 1rem;
+}
+
 .modal-details .modal-age-range {
     color: var(--text-muted);
 }
@@ -1165,12 +1169,6 @@ body {
     cursor: pointer;
 }
 
-.modal-footer {
-    padding: 1.5rem 2rem;
-    border-top: 1px solid var(--border-color);
-    display: flex;
-    justify-content: flex-end;
-}
 
 /* Buttons */
 .btn {