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>

backend/schemas.py

@@ -101,16 +101,24 @@ NUMERIC_COLUMNS = [
     "maths_avg_3yr",
 ]
 
-# School type code to name mapping
+# School type code to user-friendly name mapping
 SCHOOL_TYPE_MAP = {
+    # Academies
     "AC": "Academy",
-    "ACC": "Academy Converter",
-    "ACS": "Academy Sponsor Led",
-    "CY": "Community School",
+    "ACC": "Academy",
+    "ACCS": "Academy",
+    "ACS": "Academy (Sponsor Led)",
+    # Community Schools
+    "CY": "Community",
+    "CYS": "Community",
+    # Voluntary Schools
     "VA": "Voluntary Aided",
     "VC": "Voluntary Controlled",
+    # Foundation Schools
     "FD": "Foundation",
     "F": "Foundation",
+    "FDS": "Foundation",
+    # Free Schools
     "FS": "Free School",
 }