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`
