fix(admin): add aria-current nav indicator + fix undefined --admin-purple variable

WCAG 1.4.1 — Active nav link had no non-colour indicator in the admin panel.
Public nav already had border-bottom via common.css; admin nav had nothing.

admin.css:
- Add `[aria-current="page"]` rule on admin nav links:
    border-bottom: 2px solid currentColor; padding-bottom: 1px
  This gives a visible underline as a non-colour signal for the active page.

- Fix `--admin-purple` undefined CSS variable in pagination button hover.
  The variable was referenced but never defined in variables.css (which was
  refactored to use --accent-primary / --accent-secondary). Replaced both
  border-color and color usages with var(--accent-primary) (#9557b5 — same
  value), restoring the intended purple hover tint on pagination buttons.

todo/01-css-semantic-refactor.md:
- Audited ~15 pending CSS/HTML tasks; all were already implemented.
  Marked as done: .admin-main, .admin-page-title, .admin-form-row,
  .admin-label, .admin-input/select/textarea, .admin-table, .admin-fieldset,
  tfe.css class replacements, search.css h2 selector, admin-alert replacement,
  login.php/edit.php inline style removal, form partial hints (<small>).

todo/04-accessibility.md:
- Marked WCAG 1.4.1 admin nav and --admin-purple audit items as completed.

public/assets/css/admin.css

@@ -18,6 +18,12 @@
   opacity: 1;
 }
 
+/* Active nav link — non-colour indicator required by WCAG 1.4.1 */
+.admin-body header nav ul a[aria-current="page"] {
+  border-bottom: 2px solid currentColor;
+  padding-bottom: 1px;
+}
+
 .admin-body main {
   flex: 1;
   padding: 2.5rem 2rem 4rem;
@@ -865,8 +871,8 @@
 }
 
 .admin-body .pagination-btn:hover:not(.disabled) {
-  border-color: var(--admin-purple);
-  color: var(--admin-purple);
+  border-color: var(--accent-primary);
+  color: var(--accent-primary);
 }
 
 .admin-body .pagination-btn.disabled {