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/repertoire.css

@@ -223,6 +223,56 @@
     gap: var(--space-3xs);
 }
 
+.result-card__cover {
+    margin: 0;
+}
+
+.result-card__cover img {
+    width: 100%;
+    aspect-ratio: 4/3;
+    object-fit: cover;
+    display: block;
+    border-radius: 7px 7px 0 0;
+}
+
+.result-card__gradient {
+    width: 100%;
+    aspect-ratio: 4/3;
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    padding: var(--space-s);
+    text-align: center;
+    box-sizing: border-box;
+    border-radius: 7px 7px 0 0;
+    background: linear-gradient(
+        180deg,
+        rgba(60, 133, 108, 1) 0%,
+        rgba(51, 191, 135, 1) 25%,
+        rgba(192, 93, 225, 1) 75%,
+        rgba(149, 87, 181, 1) 100%
+    );
+}
+
+.result-card__gradient-author {
+    color: var(--accent-foreground);
+    font-size: var(--step--2);
+    opacity: 0.85;
+    margin-bottom: 0.25rem;
+    display: block;
+}
+
+.result-card__gradient-title {
+    color: var(--accent-foreground);
+    font-size: var(--step--1);
+    font-weight: 600;
+    display: -webkit-box;
+    -webkit-line-clamp: 3;
+    -webkit-box-orient: vertical;
+    overflow: hidden;
+}
+
 .result-card__authors {
     font-size: var(--step--1);
     font-weight: 500;
@@ -391,8 +441,8 @@
     background: linear-gradient(
         180deg,
         rgba(60, 133, 108, 1) 0%,
-        rgba(96, 236, 180, 1) 33%,
-        rgba(227, 144, 255, 1) 66%,
+        rgba(51, 191, 135, 1) 25%,
+        rgba(192, 93, 225, 1) 75%,
         rgba(149, 87, 181, 1) 100%
     );
     display: flex;