Add Mots-clés and Langues management to contenus page

- Add searchLanguages, getAllLanguagesWithCount, renameLanguage, mergeLanguage, deleteLanguage to Database
- Create actions/language.php handler with rename/merge/merge_bulk/delete actions
- Add merge_bulk action to actions/tag.php
- Add Mots-clés section to contenus template with HTMX search, select checkboxes, rename/delete/merge buttons, and multi-select merge toolbar
- Add Langues section to contenus template with same pattern
- Create contenus-tags-fragment.php and contenus-languages-fragment.php HTMX fragments
- Remove form-settings- from flat-fieldset CSS selector so fieldsets in contenus retain border/padding
- contenus.php: add 'Gérer les mots-clés' link to /admin/tags.php
- contenus.php: add Langues fieldset with HTMX search + table (rename/merge/delete/bulk)
- tags.php: add HTMX search bar, checkbox column, bulk merge toolbar
- Create tags-fragment.php and contenus-langues-fragment.php for HTMX
- Remove tab component and associated CSS
- Simplify JS: separate tags/langues-prefixed functions
- Fix redirects: tag.php defaults to /admin/tags.php, supports return override
- Keep tags.php standalone page and Mots-clés button unchanged

app/public/assets/css/admin.css

@@ -615,16 +615,14 @@ th.admin-ap-col {
     margin-bottom: var(--space-xl);
 }
 
-/* Fieldsets inside flat sections: no card border */
-.admin-body main > section[aria-labelledby^="settings-"] fieldset,
-.admin-body main > section[aria-labelledby^="form-settings-"] fieldset {
+/* Fieldsets inside flat settings sections: no card border */
+.admin-body main > section[aria-labelledby^="settings-"] fieldset {
     border: none;
     border-radius: 0;
     padding: var(--space-m) 0;
 }
 
-.admin-body main > section[aria-labelledby^="settings-"] fieldset legend,
-.admin-body main > section[aria-labelledby^="form-settings-"] fieldset legend {
+.admin-body main > section[aria-labelledby^="settings-"] fieldset legend {
     padding: 0;
     font-weight: 600;
     letter-spacing: 0.04em;
@@ -633,8 +631,7 @@ th.admin-ap-col {
 }
 
 .admin-body main > section[aria-labelledby^="settings-"] > h2,
-.admin-body main > section[aria-labelledby^="static-pages-"] > h2,
-.admin-body main > section[aria-labelledby^="form-settings-"] > h2 {
+.admin-body main > section[aria-labelledby^="static-pages-"] > h2 {
     font-weight: 600;
     letter-spacing: 0.06em;
     text-transform: uppercase;
@@ -1992,3 +1989,56 @@ th.admin-ap-col {
   50.01% { transform: scaleX(1); transform-origin: right; }
   100% { transform: scaleX(0);   transform-origin: right; }
 }
+
+/* ── Sidebar TOC ───────────────────────────────────────────────────────────── */
+
+.admin-with-toc {
+    display: flex;
+    gap: var(--space-m);
+    align-items: flex-start;
+    max-width: var(--content-max-width, 1200px);
+    margin: 0 auto;
+    padding: 0 var(--space-s);
+}
+
+.admin-with-toc > main {
+    flex: 1;
+    min-width: 0;
+}
+
+.admin-toc {
+    position: sticky;
+    top: var(--space-m);
+    width: 160px;
+    flex-shrink: 0;
+    padding-top: var(--space-s);
+}
+
+.admin-toc-list {
+    list-style: none;
+    margin: 0;
+    padding: 0;
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+}
+
+.admin-toc-list a {
+    display: block;
+    padding: var(--space-3xs) var(--space-2xs);
+    font-size: var(--step--2);
+    color: var(--text-secondary);
+    text-decoration: none;
+    border-left: 2px solid transparent;
+    transition: color 0.15s, border-color 0.15s;
+}
+
+.admin-toc-list a:hover {
+    color: var(--text-primary);
+}
+
+.admin-toc-list a.admin-toc-active {
+    color: var(--text-primary);
+    font-weight: 600;
+    border-left-color: var(--accent, var(--color-primary));
+}