Replace HTMX+PHP file upload queues with client-side JS

Drops the session-backed HTMX incremental upload system in favour of a
single JS module that manages `File` objects client-side and injects
them into `FormData` on submit.

Key changes:

* `file-upload-queue.js`: client-side queues with validation, reorder
  (SortableJS), removal, dirty-state tracking, and fetch-based submit
  with manual redirect handling
* `fichiers-fragment.php`: empty queue containers for JS-managed queues;
  HTMX format switching still works with queue rehydration after swap;
  annexe uploads now support multiple files
* Form UI cleanup: moved existing files and cover preview into the
  `Fichiers` fieldset (edit mode); removed redundant queue labels while
  keeping labels for single-file inputs (`couverture`,
  `note d'intention`); added delete buttons for existing files
* `ThesisFileHandler.php`: added
  `handleTfeQueueFiles()`/`handleAnnexeQueueFiles()` reading from
  `$_FILES['queue_file']`; introduced `extractFilesSubArray()` for
  nested upload arrays; removed session-based queue handling
* `ThesisCreateController.php` &
  `ThesisEditController.php`: switched to extracted
  `['queue_file']` uploads
* `beforeunload-guard.js`: now also watches
  `window.__xamxamDirty`
* Deleted obsolete PHP upload/remove/reorder queue endpoints for
  `partage` and `admin`
* Cleaned up route dispatch in `partage/index.php`
* Misc form and styling updates in templates/CSS
* Added `docs/cms-migration-plan.html`

app/templates/partials/form/fieldset-files.php

@@ -48,7 +48,7 @@ $adminMode = $adminMode ?? false;
 
             <!-- File queue — populated by JS -->
             <ul id="tfe-file-queue" class="tfe-file-queue" aria-label="Fichiers sélectionnés">
-                <!-- Queue rendered server-side via HTMX (upload-tfe-file) -->
+                <!-- File queue rendered client-side by file-upload-queue.js -->
             </ul>
             <p id="tfe-file-queue-empty" class="tfe-queue-empty">Aucun fichier sélectionné.</p>
         </div>