fix: restore dark theme tokens for login/admin/legal pages + remove formation SEO

- globals.css: restore missing dark-*, primary, error, warning color tokens
  and gradient-text/gradient-bg utility classes so login, admin, mentions-legales,
  and confidentialite pages render correctly (white text was invisible on white bg)
- layout.tsx: remove "Candidature Formation" from JSON-LD SiteNavigationElement
- candidature/layout.tsx: remove "Formation" from title/description, add noindex
- robots.ts: disallow /candidature/ from crawlers
- sitemap.ts: remove /candidature from sitemap

https://claude.ai/code/session_01V8YAjpqRQ3bfBYsABYsEgo

app/layout.tsx

@@ -193,11 +193,6 @@ export default function RootLayout({
         name: "D\u00e9mo Plombier",
         url: `${BASE_URL}/plombier`,
       },
-      {
-        "@type": "WebPage",
-        name: "Candidature Formation",
-        url: `${BASE_URL}/candidature`,
-      },
       {
         "@type": "WebPage",
         name: "Mentions L\u00e9gales",