fix: resolve partage form submission issues

- Replace mb_strlen/mb_substr/mb_strtolower with strlen/substr/strtolower
  (mbstring extension missing on server, causing fatal error)
- Scope annexes checkbox HTMX swap to #annexes-input-block with hx-select
  (prevents duplicating entire page inside Fichiers fieldset)
- Split format+fichiers response: #format-fichiers-block (stable) and
  #format-extras-block (swappable, inside Fichiers fieldset). Format
  checkboxes use hx-select to extract only the extras, preserving file queue.
- Keep format extras inline in Fichiers fieldset (no sub-fieldsets). Remove
  website legend input (URL only).
- When PeerTube upload disabled, show direct file upload inputs for
  video/audio (name=files[]).
- Add "Glissez-déposez" sort hint below TFE file queue.
- Fix .fq-name overflow with width:0;min-width:100% chain.
- Remove legend placeholder from .fq-item.
- Merge "Récits et expérimentation" AP into "Narration Spéculative".
  Rename PACS to "Pratique de lart - outils critiques, arts et contexte
  simultanés".
- Remove président·e field from jury fieldset, form templates, and
  controller validation. Keep DB column and display logic for existing data.

app/src/Controllers/ThesisCreateController.php

@@ -427,10 +427,6 @@ class ThesisCreateController
                 }
             }
         }
-        if (!empty(trim($post['jury_president'] ?? ''))) {
-            $juryMembers[] = ['name' => trim($post['jury_president']), 'role' => 'president', 'is_external' => 0];
-        }
-
         if (!$adminMode && !$hasPromoteur) {
             throw new Exception('Veuillez indiquer au moins un·e promoteur·ice interne.');
         }
@@ -508,8 +504,8 @@ class ThesisCreateController
 
         // Note contextuelle (optional, max 1500 chars)
         $contextNote = $this->sanitiseString($post['context_note'] ?? '');
-        if (mb_strlen($contextNote) > 1500) {
-            $contextNote = mb_substr($contextNote, 0, 1500);
+        if (strlen($contextNote) > 1500) {
+            $contextNote = substr($contextNote, 0, 1500);
         }
 
         // Backoffice fields (admin only)