STAGE 73 — PWA ACCOUNT & SECURITY CENTER

Adds a protected Account & Security section to the PWA.

Features:
- role-aware account summary using existing profile/permissions APIs
- password change through the existing authenticated password API
- client-side validation (10+ characters and confirmation)
- sign out using the existing memory-only auth lifecycle
- no localStorage/sessionStorage/indexedDB use for account data
- service worker cache bumped to v28 for new public PWA assets
- API/private routes remain excluded from service-worker caching

TEST:
Open /school-management-system/upgrade/stage73.php
All checks must show PASS, then open the PWA shell and sign in.

Do not test a real password change unless you intentionally want to change the account password. The stage UI is safe to inspect without submitting the form.
