Unify form CSS: move licence/share-badge styles to admin.css, remove inline style from partage form

TODO.md

@@ -7,3 +7,4 @@
 - [x] Add SMTP test email button in parametres.php (action + CSS)
 - [x] Fix SmtpRelay::send() — replace broken mail() stub with native PHP socket SMTP client (STARTTLS/SSL, AUTH PLAIN/LOGIN)
 - [x] Lock body scroll on all pages (admin + public); only `main` or inner element scrolls
+- [x] Unify form CSS between admin `add.php` and student partage form: move licence/share-badge styles into `admin.css`, remove inline `<style>` from `partage/index.php`, switch to `common.css` + `admin.css`