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/package-lock.json

@@ -8,6 +8,7 @@
       "name": "frontend",
       "version": "0.1.0",
       "dependencies": {
+        "@codegouvfr/react-dsfr": "^1.32.4",
         "@tanstack/react-query": "^5.90.21",
         "axios": "^1.13.6",
         "lucide-react": "^0.577.0",
@@ -370,6 +371,29 @@
         "specificity": "bin/cli.js"
       }
     },
+    "node_modules/@codegouvfr/react-dsfr": {
+      "version": "1.32.4",
+      "resolved": "https://registry.npmjs.org/@codegouvfr/react-dsfr/-/react-dsfr-1.32.4.tgz",
+      "integrity": "sha512-yhjPqPObB3BsehZTRhJZ9qXwBRT2RZl0cn6hG838MlkPntTE5H5w0vU05KmKcdDBnQ0wDV59G7nnZrpKZQlNuA==",
+      "license": "MIT",
+      "dependencies": {
+        "tsafe": "^1.8.5",
+        "yargs-parser": "^21.1.1"
+      },
+      "bin": {
+        "copy-dsfr-to-public": "bin/copy-dsfr-to-public.js",
+        "only-include-used-icons": "bin/only-include-used-icons.js",
+        "react-dsfr": "bin/react-dsfr.js"
+      },
+      "peerDependencies": {
+        "@gouvfr/dsfr-chart": "^1.0.0"
+      },
+      "peerDependenciesMeta": {
+        "@gouvfr/dsfr-chart": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/@csstools/color-helpers": {
       "version": "6.0.2",
       "resolved": "https://registry.npmjs.org/@csstools/color-helpers/-/color-helpers-6.0.2.tgz",
@@ -8001,6 +8025,12 @@
         "typescript": ">=4.8.4"
       }
     },
+    "node_modules/tsafe": {
+      "version": "1.8.12",
+      "resolved": "https://registry.npmjs.org/tsafe/-/tsafe-1.8.12.tgz",
+      "integrity": "sha512-nFRqW0ttu/2o6XTXsHiVZWJBCOaxhVqZLg7dgs3coZNsCMPXPfwz+zPHAQA+70fNnVJLAPg1EgGIqK9Q84tvAw==",
+      "license": "MIT"
+    },
     "node_modules/tsconfig-paths": {
       "version": "3.15.0",
       "resolved": "https://registry.npmjs.org/tsconfig-paths/-/tsconfig-paths-3.15.0.tgz",
@@ -8964,6 +8994,15 @@
       "dev": true,
       "license": "ISC"
     },
+    "node_modules/yargs-parser": {
+      "version": "21.1.1",
+      "resolved": "https://registry.npmjs.org/yargs-parser/-/yargs-parser-21.1.1.tgz",
+      "integrity": "sha512-tVpsJW7DdjecAiFpbIB1e3qxIQsE6NoPc5/eTdrbbIC4h0LVsWhnoa3g+m2HclBIujHzsxZ4VJVA+GUuc2/LBw==",
+      "license": "ISC",
+      "engines": {
+        "node": ">=12"
+      }
+    },
     "node_modules/yocto-queue": {
       "version": "0.1.0",
       "resolved": "https://registry.npmjs.org/yocto-queue/-/yocto-queue-0.1.0.tgz",