Fix WCAG 4.1.2 truncated select text + split admin/public favicons

- admin/edit.php: remove mb_strimwidth(60) truncation from access_type
  <select> option labels; full 'name — description' text is now the
  accessible name so screen readers get unambiguous option text (WCAG 4.1.2)

- public/assets/favicon.svg: new public favicon — brand-purple (#9557b5)
  rounded square with white 'P' lettermark; distinct from admin_favicon.svg
  (archive-restore Lucide icon in #c104fc) which is admin-only

- templates/head.php: favicon <link> now conditionally serves favicon.svg
  (public pages) or admin_favicon.svg (admin pages) based on $isAdmin;
  closes the open favicon task in todo/01-css-semantic-refactor.md

- todo/04-accessibility.md: mark WCAG 3.1.1 lang audit and WCAG 4.1.2
  select truncation items as done
- todo/01-css-semantic-refactor.md: mark favicon task as done

public/assets/favicon.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="24" height="24">
+  <rect width="24" height="24" rx="4" fill="#9557b5"/>
+  <text x="12" y="17.5" font-family="Georgia, serif" font-size="15" font-weight="bold"
+        fill="#ffffff" text-anchor="middle">P</text>
+</svg>