Refactor apropos/charte/licence pages: shared layout, TOC anchors, and UI polish

Unify the three public pages (à propos, charte, licence) onto a single
grid layout (.page-content) with sticky TOC sidebar, replacing the old
separate  /  /  markup.

- Merge about.php, charte.php, licence.php templates into shared
  .page-content / .content-section structure
- Add CommonMark HeadingPermalinkExtension for stable heading anchors
- Use SlugNormalizer for TOC links so they match rendered heading IDs
- Standardize link styling across content blocks: bold black, accent on
  hover (consistent with global link style)
- Fix code block wrapping: use pre-wrap instead of pre, constrain grid
  columns with min-width:0, auto scrollbar
- Fix apropos page grid placement: force content-section into column 2
  so contacts and credits stay in the content area, not the sidebar

Also includes accumulated WIP changes:
- Header gradient: hardcoded purple-to-green (replaces CSS variables)
- Search placeholder font
- Duration field: replace minutes/sec/heures with h:m:s time inputs
- TFE file optional for formats 1,4,6 with client-side JS toggle
- Licence form: em-dash to hyphen, details/summary classes
- Pill search: block Enter key form submission when no results
- Draft autosave: remove CSRF rotation (broke concurrent FilePond uploads)
- Language pill: clear hints for excluded main languages
- Search results: gradient placeholder cards for items without covers
- TFE display: format durée values as XhYm instead of decimal

app/public/assets/css/components/header.css

@@ -8,12 +8,13 @@
 header {
   vertical-align: center;
   flex-shrink: 0;
+  background: #9557B5;
   background: linear-gradient(
-    180deg,
-    var(--gradient-1) 0%,
-    var(--gradient-2) 33%,
-    var(--gradient-3) 66%,
-    var(--gradient-4) 100%
+    0deg,
+    rgba(149, 87, 181, 1) 0%,
+    rgba(192, 93, 225, 1) 25%,
+    rgba(51, 191, 135, 1) 75%,
+    rgba(60, 133, 108, 1) 100%
   );
 }
 
@@ -60,7 +61,6 @@ header nav ul a[aria-current="page"] {
   padding-bottom: 1px;
 }
 
-
 /* ── Logo ───────────────────────────────────────────────────────────── */
 
 .nav-logo {
@@ -130,8 +130,12 @@ header nav ul a[aria-current="page"] {
   transition: all 0.3s ease-out;
 }
 
-.navicon::before { top: -7px; }
-.navicon::after  { bottom: -7px; }
+.navicon::before {
+  top: -7px;
+}
+.navicon::after {
+  bottom: -7px;
+}
 
 /* ── Mobile (≤ 640px) ───────────────────────────────────────────────── */
 
@@ -160,11 +164,14 @@ header nav ul a[aria-current="page"] {
   }
 
   header nav[aria-label="Navigation principale"]
-    .nav-left-links li:not(:first-child) {
+    .nav-left-links
+    li:not(:first-child) {
     display: none;
   }
 
-  .menu-icon { display: flex; }
+  .menu-icon {
+    display: flex;
+  }
 
   header nav[aria-label="Navigation principale"] .nav-mobile-links {
     display: block;