refactor: session-based incremental TFE upload via HTMX, drop SortableJS

Replace the client-side FileArray + Sortable drag-to-reorder with a
server-side session-based upload flow:

- New endpoints: /partage/upload-tfe-file, /partage/remove-tfe-file
  (and /admin/ variants) — single-file incremental upload via HTMX
  multipart/form-data with progress bar support
- Session storage: uploaded files go to STORAGE_ROOT/uploads/{session_id}/
  with metadata in $_SESSION['tfe_uploads']
- file-upload-queue.js reduced to single-file previews only (couverture,
  note_intention, annexes thumbnails)
- ThesisFileHandler gains handleTfeFilesFromSession + writeTfeFileFromSrc
  + cleanupSessionUploads for final commit from session temp
- Sortable.min.js removed from all script tags; drag handles and ghost
  CSS removed
- No file_orders[]/file_labels[] hidden field injection needed
- Upload queue survives page refresh (server-owned list)

This eliminates the SortableJS dependency entirely while keeping the
same UX: pick files, see them in a queue, remove individual files.

app/templates/partials/form/form.php

@@ -31,7 +31,7 @@
  *   bool   $showFlash           — render flash banners (error/warning/success)
  *   bool   $showContact         — Contact checkbox fieldset
  *   bool   $showCoverPreview    — cover image preview + remove checkbox
- *   bool   $showExistingFiles   — existing thesis files list (sortable, deletable)
+ *   bool   $showExistingFiles   — existing thesis files list (deletable)
  *   bool   $showBackoffice      — Backoffice fieldset (context_note, jury_points, remarks, baiu_link, exemplaires, contact_interne, is_published)
  *   bool   $showEmailConfirmation — E-mail de confirmation fieldset
 
@@ -340,7 +340,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
 
         <!-- Edit-only: existing files management -->
         <div id="edit-existing-files-block">
-            <!-- Existing thesis files — sortable, with labels -->
+            <!-- Existing thesis files — with labels -->
             <?php $thesisFilesList = array_values(
                 array_filter(
                     $currentFiles,
@@ -350,10 +350,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
             <?php if (!empty($thesisFilesList)): ?>
             <div class="admin-form-group">
                 <label>Fichiers du TFE existants :</label>
-                <small style="display:block;margin-bottom:var(--space-2xs);color:var(--text-tertiary)">
-                    Glissez-déposez les lignes pour réordonner les fichiers sur la page publique.
-                </small>
-                <ul id="existing-files-sortable" class="admin-file-list sortable-list">
+                <ul id="existing-files-sortable" class="admin-file-list">
                     <?php foreach ($thesisFilesList as $f):
 
                         $fExt = strtolower(
@@ -400,7 +397,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
                         <input type="hidden" name="file_sort_order[]" value="<?= (int) $f[
                             "id"
                         ] ?>">
-                        <span class="admin-file-drag-handle" title="Réordonner">⠿</span>
+
                         <span class="admin-file-icon-col"><?= $fIcon ?></span>
                         <span class="admin-file-info">
                             <a href="<?= $fLinkHref ?>" target="_blank" rel="noopener" class="admin-file-name">