edit.php: rework Fichiers fieldset layout

- Drop file-field.php partial for cover/banner (it added a second label)
- Inline all three file inputs with admin-file-input wrapper
- Move banner inside the Fichiers fieldset
- Each entry: one label, one input, one small hint — no duplicate labels
- Context-aware hints: 'Laisser vide pour conserver...' when file already exists

TODO.md

@@ -7,6 +7,7 @@
 - [x] Add `deleteThesisFile()` and `handleCoverUpload()` to Database.php
 - [x] Update `ThesisEditController::save()` to handle cover, file deletion, new uploads
 - [x] Update `ThesisEditController::load()` to expose `currentFiles` + `currentCover`
+- [x] Rework Fichiers fieldset layout: inline file inputs (no partial double-label), banner moved inside fieldset, one label per field
 
 
 ## Overview