School Branding Registered-Schools Patch

Changes:
- Super Administrator School Branding now reads the registered schools list.
- Added a Registered School selector to the School Branding panel.
- Selecting a school loads that school's saved branding settings.
- Saving branding explicitly targets the selected registered school.
- The server validates the selected school before reading or writing branding.
- Ordinary school administrators remain restricted to their assigned school.
- Public school portals continue to load branding by school ID/slug, so saved logo, colors, name, motto and contact details take effect on the correct school portal.