fix: mark languages as required, add required-field visual indicators on both forms

- checkbox-list.php: support $required prop → adds required + aria-required on fieldset
- add.php: languages checkbox now marked required (matches server-side validation)
- partage/index.php: same for student form
- admin.css: dashed border on required inputs, bold labels, red asterisk via :has(), "Champs obligatoires" note
- Both forms now show "* Champs obligatoires" note at top

Server-side required fields = titre, auteurice, synopsis, année, orientation, ap, finality, languages (≥1), access_type_id, confirmation_email. All now have required attribute + visual asterisk.

docs/CSS.md

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+# CSS Architecture
+
+## File Structure
+
+- **variables.css** — all CSS custom properties (single source of truth for every color/token)
+- **common.css** — reset, header/nav, search bar, accessibility utilities (loaded on all pages)
+- **main.css** — home page
+- **search.css** — search/directory page
+- **tfe.css** — individual thesis page
+- **apropos.css** — about + licence pages
+- **system.css** — admin system dashboard
+- **admin.css** — admin section (loaded alongside `common.css` on every admin page)
+- **modern-normalize.min.css** — third-party reset (minified, do not edit)
+
+## Rules
+
+- Every color value lives in `variables.css` as a CSS custom property.
+- No hardcoded hex, rgb(), or rgba() in any other file.
+- All files `@import url("./variables.css")` at the top.
+- Admin and public share the same token names — no separate admin theme.
+- No dark-mode media query. System page uses the same light tokens as the rest of the admin section.