feat: prefill the mairie form from the picked event

Wire the seam end to end: the background mirrors config.local.json into
storage (a content script would need web_accessible_resources, which would
expose the email to the mairie page), and the content script lays the computed
values into the form.

The layer is deliberately thin and untested: all the logic lives in
formulaire-valeurs.js. It never writes an empty value, never touches the image,
the honeypots or the CSRF tokens, and never submits: a human reads and sends.
A banner states what is left to do, and warns when a field could not be filled,
so that a silent failure stays diagnosable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/manifest.json

@@ -10,8 +10,18 @@
     }
   },
   "background": {
-    "scripts": ["background.js"]
+    "scripts": ["config.js", "background.js"]
   },
+  "content_scripts": [
+    {
+      "matches": [
+        "https://www.ville-cugnaux.fr/mes-loisirs/associations/proposer-un-evenement-dans-lagenda/*"
+      ],
+      "js": ["config.js", "formulaire-valeurs.js", "formulaire-mairie.js"],
+      "css": ["formulaire-mairie.css"],
+      "run_at": "document_idle"
+    }
+  ],
   "action": {
     "default_title": "Ouvrir la liste de l'Écho du Huit",
     "default_icon": "icons/icon.svg"