feat(codev): M3 - CodevGraph D3 force-directed + page carto affichage

- Install d3@^7.9.0 (absent du projet, requis pour force simulation)
- components/codev/CodevGraph.vue : simulation forceLink/forceManyBody/forceCenter/forceCollide, drag D3, pastilles offre (vert) + besoin (orange), tooltip SVG natif, ResizeObserver, watch matches/mode pret pour M4, placeholder si 0 fiches
- pages/codev/carto.vue : useFetch /api/codev/fiches, mount CodevGraph, refs matches+mode vides (M4 les remplira)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -13,6 +13,7 @@
     "@headlessui/vue": "^1.7.23",
     "@types/leaflet": "^1.9.21",
     "@types/leaflet.markercluster": "^1.5.6",
+    "d3": "^7.9.0",
     "ioredis": "^5.3.2",
     "leaflet": "^1.9.4",
     "leaflet.markercluster": "^1.5.3",