Remove dead template/asset files; fix licence.php full-width layout

- Delete templates/header.php and templates/head.php — both were legacy
  partials from a previous design iteration (lang="en", broken nav markup)
  that were never included anywhere in the current codebase.

- Delete public/assets/icons.svg — the full TrumboWYG icon sprite (~15 KB)
  referenced nowhere; the only active WYSIWYG editor (EasyMDE in
  pages-edit.php) loads its own assets from CDN.

- Fix licence.php layout: the page was borrowing the two-column
  .apropos-layout grid but leaving the right column always empty, wasting
  ~40% of the viewport. Removed the grid wrapper and the empty .apropos-right
  div. Added .apropos-single utility class to apropos.css (max-width: 720px)
  so licence content now spans the full available width with a readable
  line length.

TODO.md

@@ -463,12 +463,12 @@ Goal: rename the tables and column to the canonical M2M pattern (`tags`, `thesis
       `apropos.php`, `licence.php`, `templates/admin/head.php` all contain the same 6-line
       `(function poll(){…})()` block. Consolidate into the shared head partials.
 
-- [ ] **`templates/header.php` and `templates/head.php` are dead files** — neither is `include`d
+- [x] **`templates/header.php` and `templates/head.php` are dead files** — neither is `include`d
       anywhere in the codebase. Both contain outdated markup from a previous design iteration
       (`lang="en"`, empty author meta, a broken nav with double-quoted `href` attributes inside
       `href`). Delete both to remove confusion.
 
-- [ ] **`public/assets/icons.svg` is dead** — it is the full TrumboWYG editor icon sprite (40+
+- [x] **`public/assets/icons.svg` is dead** — it is the full TrumboWYG editor icon sprite (40+
       symbols) referenced **nowhere** in the codebase. The only WYSIWYG editor in use (EasyMDE
       in `pages-edit.php`) loads from CDN. Delete `icons.svg` (~15 KB of noise).
 
@@ -525,10 +525,10 @@ Goal: rename the tables and column to the canonical M2M pattern (`tags`, `thesis
       require a code deploy to change. Either move them into the `about` page Markdown (admin-
       editable) or extract to a config array so they are in one place.
 
-- [ ] **`licence.php` wastes half the viewport with an always-empty right column** — the page
+- [x] **`licence.php` wastes half the viewport with an always-empty right column** — the page
       reuses the two-column `.apropos-layout` but `<div class="apropos-right"></div>` is always
-      empty. Add a `.apropos-layout--single` variant (or just `grid-template-columns:1fr` when
-      the right child is empty) to use the full width for content.
+      empty. Added `.apropos-single` class + removed layout wrapper; content now uses full width
+      (max-width: 720px) without an empty right column.
 
 ### G — Accessibility & semantics