feat(frontend): socle de refonte au Système de Design de l'État (DSFR)

- charge le DSFR (CSS/JS officiels) en statique via public/dsfr + postinstall
- en-tête officiel (bloc Marianne « République Française » + nom de service)
- pied de page officiel DSFR (liens institutionnels, licence etalab)
- page d'accueil refondue (hero, fr-card, fr-tile, fr-callout, fr-btn)
- composant Button mappé sur fr-btn
- assets DSFR hors git (régénérés par `npm install`)

Build OK (19 routes). Pages internes encore en Tailwind : conversion à poursuivre.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/ui/Button.tsx

@@ -11,23 +11,19 @@ interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
   loading?: boolean;
 }
 
+// Mappe les variantes internes sur les classes du Système de Design de l'État (DSFR).
 const variantClasses: Record<ButtonVariant, string> = {
-  primary:
-    "bg-candlelight-500 hover:bg-candlelight-600 text-black font-semibold shadow-sm hover:shadow-md",
-  secondary:
-    "bg-malachite-500 hover:bg-malachite-600 text-white font-semibold shadow-sm hover:shadow-md",
-  outline:
-    "border-2 border-candlelight-500 text-candlelight-700 dark:text-candlelight-400 hover:bg-candlelight-50 dark:hover:bg-candlelight-950",
-  ghost:
-    "hover:bg-gray-100 dark:hover:bg-gray-800 text-gray-700 dark:text-gray-300",
-  danger:
-    "bg-red-500 hover:bg-red-600 text-white font-semibold shadow-sm hover:shadow-md",
+  primary: "",
+  secondary: "fr-btn--secondary",
+  outline: "fr-btn--secondary",
+  ghost: "fr-btn--tertiary-no-outline",
+  danger: "fr-btn--secondary",
 };
 
 const sizeClasses: Record<ButtonSize, string> = {
-  sm: "px-3 py-1.5 text-sm",
-  md: "px-5 py-2.5 text-base",
-  lg: "px-7 py-3.5 text-lg",
+  sm: "fr-btn--sm",
+  md: "",
+  lg: "fr-btn--lg",
 };
 
 export default function Button({
@@ -39,43 +35,18 @@ export default function Button({
   className = "",
   ...rest
 }: ButtonProps) {
+  const classes = [
+    "fr-btn",
+    variantClasses[variant],
+    sizeClasses[size],
+    loading ? "fr-btn--icon-left fr-icon-refresh-line" : "",
+    className,
+  ]
+    .filter(Boolean)
+    .join(" ");
+
   return (
-    <button
-      disabled={disabled || loading}
-      className={`
-        inline-flex items-center justify-center gap-2
-        rounded-xl transition-all duration-200
-        focus:outline-none focus:ring-2 focus:ring-candlelight-500 focus:ring-offset-2
-        dark:focus:ring-offset-gray-900
-        disabled:opacity-50 disabled:cursor-not-allowed
-        ${variantClasses[variant]}
-        ${sizeClasses[size]}
-        ${className}
-      `}
-      {...rest}
-    >
-      {loading && (
-        <svg
-          className="animate-spin h-4 w-4"
-          xmlns="http://www.w3.org/2000/svg"
-          fill="none"
-          viewBox="0 0 24 24"
-        >
-          <circle
-            className="opacity-25"
-            cx="12"
-            cy="12"
-            r="10"
-            stroke="currentColor"
-            strokeWidth="4"
-          />
-          <path
-            className="opacity-75"
-            fill="currentColor"
-            d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"
-          />
-        </svg>
-      )}
+    <button className={classes} disabled={disabled || loading} {...rest}>
       {children}
     </button>
   );