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/form-base.css

@@ -352,6 +352,34 @@
   border-radius: var(--radius);
 }
 
+.duration-time-inputs {
+  display: flex;
+  align-items: flex-end;
+  gap: var(--space-xs);
+}
+
+.duration-time-fields {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-3xs);
+}
+
+.duration-time-fields span {
+  font-size: var(--step--1);
+  color: var(--text-secondary);
+}
+
+.licence-details {
+  display: inline-flex;
+}
+
+.licence-summary {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-3xs);
+  cursor: pointer;
+}
+
 .licence-degree h4 {
   margin: 0 0 var(--space-2xs);
   font-weight: 600;