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/ThesisEditController.php

@@ -650,15 +650,6 @@ class ThesisEditController
             }
         }
 
-        // President (optional, admin-only)
-        if (!empty(trim($post['jury_president'] ?? ''))) {
-            $members[] = [
-                'name'        => trim($post['jury_president']),
-                'role'        => 'president',
-                'is_external' => 0,
-            ];
-        }
-
         // Backwards compat: old jury_lecteurs[]
         if (isset($post['jury_lecteurs'])) {
             foreach ($post['jury_lecteurs'] as $i => $name) {