Login form is now email+password (was a bare shared password field). Nav conditionally
shows Instructors (owner only) and Audit (owner/admin) tabs based on the signed-in
instructor's role. New InstructorsView (create/list/edit accounts, role/status controls,
session list + revoke-all, password reset — owner only), AuditView (read-only audit event
table), AccountView (change-password form for the signed-in instructor). CohortsView hides
the create-cohort form and curriculum-assignment button for the 'instructor' role — the
server already enforces this; this just avoids showing controls that would 403.
