Add sidebar TOC, simplify Données Secondaires section

- Rename 'Éditer Données Secondaires' → 'Données Secondaires', remove fieldset wrapper on Mots-clés link
- Create admin-toc.php partial: IntersectionObserver-based sidebar nav
- Include TOC on contenus.php, acces.php, parametres.php
- Add .admin-with-toc flex layout (sidebar + main) and .admin-toc CSS
- Fonts (Ductus, BBB DM Sans): verified loaded via variables.css → common.css import chain
- TOC: move inside <main> as <aside>, content in <article>, fix scrolling
- Lazy load: hx-trigger='load delay:100ms' with spinner (htmx-indicator) for tags/langues
- Inline rename: edit button in Nom cell, HTMX post for rename, validate+ cancel buttons
- Checkbox column: width:1% / fit-content
- Remove per-row merge forms/selects, only bulk merge when ≥2 checkboxes selected
- Remove per-row merge dialogs, keep only bulk merge and delete dialogs
- Add htmx-settling CSS transition for lazy-load fade-in
- Update acces.php/parametres.php: article layout, TOC inside main
- TOC: DOMContentLoaded guard, use <nav>+<a> directly instead of <ul>/<li>
- Section spacing: margin-bottom on sections and fieldsets in admin-main--toc
- Language dedup: GROUP BY LOWER(name) in getAllLanguagesWithCount and searchLanguages
- deduplicateLanguages() merges duplicate names and reassigns thesis_languages
- Sticky bulk-actions: position:sticky;top:0;z-index:10
- Tags toolbar: title left, stat count right (margin-left:auto), search bar under
- Tags count stat updated via hx-swap-oob from fragment
- Remove margin/max-width from .admin-main--toc
- Gap between TOC and article: --space-xs, sticky top: --space-xs
- Main padding: --space-s / --space-m / --space-xl (was --space-l/--space-l/--space-2xl)
- Article padding-top: --space-m

app/public/assets/css/admin.css

@@ -48,7 +48,7 @@
     flex: 1;
     min-height: 0;
     overflow-y: auto;
-    padding: var(--space-l) var(--space-l) var(--space-2xl);
+    padding: var(--space-s) var(--space-m) var(--space-xl);
     width: 100%;
     background: linear-gradient(
         180deg,
@@ -331,6 +331,9 @@
     border-radius: var(--radius);
     margin-bottom: var(--space-s);
     font-size: var(--step--1);
+    position: sticky;
+    top: 0;
+    z-index: 10;
 }
 
 .admin-bulk-btns {
@@ -338,6 +341,14 @@
     gap: var(--space-2xs);
 }
 
+.admin-stat--inline {
+    font-size: var(--step--1);
+    color: var(--text-secondary);
+    font-weight: 500;
+    white-space: nowrap;
+}
+}
+
 /* ── Table ──────────────────────────────────────────────────────────────── */
 /* Base table/th/td styles live in common.css */
 .admin-body main > table {
@@ -1992,26 +2003,32 @@ th.admin-ap-col {
 
 /* ── Sidebar TOC ───────────────────────────────────────────────────────────── */
 
-.admin-with-toc {
+.admin-main--toc {
     display: flex;
-    gap: var(--space-m);
+    gap: var(--space-xs);
     align-items: flex-start;
-    max-width: var(--content-max-width, 1200px);
-    margin: 0 auto;
-    padding: 0 var(--space-s);
 }
 
-.admin-with-toc > main {
+.admin-main--toc > article {
     flex: 1;
     min-width: 0;
+    padding-top: var(--space-m);
+}
+
+.admin-main--toc > article > section {
+    margin-bottom: var(--space-xl);
+}
+
+.admin-main--toc > article > section > fieldset {
+    margin-bottom: var(--space-m);
 }
 
 .admin-toc {
     position: sticky;
-    top: var(--space-m);
+    top: var(--space-xs);
     width: 160px;
     flex-shrink: 0;
-    padding-top: var(--space-s);
+    padding-top: var(--space-m);
 }
 
 .admin-toc-list {
@@ -2042,3 +2059,11 @@ th.admin-ap-col {
     font-weight: 600;
     border-left-color: var(--accent, var(--color-primary));
 }
+
+/* ── Lazy-load transition ─────────────────────────────────────────────────── */
+.htmx-settling img {
+    opacity: 0;
+}
+.htmx-indicator img {
+    transition: opacity 300ms ease-in;
+}