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/templates/admin/add.php

@@ -16,8 +16,6 @@
     $juryPromoteursUlb = [];
     $lecteursInternes = [];
     $lecteursExternes = [];
-    $juryPresident    = null;
-    $showPresident    = false;
     $showPromoteurUlb = true;
     $promoteurUlbConditional = false;