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/public/assets/js/beforeunload-guard.js

@@ -2,6 +2,7 @@
  * Beforeunload guard — prompts the user before navigating away from unsaved changes.
  *
  * Attach to any form with a data-beforeunload-guard attribute.
+ * Also watches window.__xamxamDirty (set by file-upload-queue.js).
  * No effect when JavaScript is unavailable (form posts normally).
  */
 (() => {
@@ -13,11 +14,11 @@
     for (const form of forms) {
         form.addEventListener('input', () => { dirty = true; });
         form.addEventListener('change', () => { dirty = true; });
-        form.addEventListener('submit', () => { dirty = false; });
+        form.addEventListener('submit', () => { dirty = false; window.__xamxamDirty = false; });
     }
 
     window.addEventListener('beforeunload', (e) => {
-        if (dirty) {
+        if (dirty || window.__xamxamDirty) {
             e.preventDefault();
         }
     });