feat: scaffold Firefox extension skeleton with manifest validation tests

Manifest V3 (Firefox keys: background.scripts, host_permissions), a
background event page that opens a standalone list page on toolbar
click, and the placeholder list page (HTML/CSS/JS vanilla). No build
step. extension.test.ts validates manifest shape and referenced files.

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

extension/manifest.json

@@ -0,0 +1,24 @@
+{
+  "manifest_version": 3,
+  "name": "Écho du Huit",
+  "version": "0.1.0",
+  "description": "Liste autonome agrégeant les infos de l'Atelier du Huit et de la ville de Cugnaux.",
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "echo-du-huit@atelier-huit",
+      "strict_min_version": "127.0"
+    }
+  },
+  "background": {
+    "scripts": ["background.js"]
+  },
+  "action": {
+    "default_title": "Ouvrir la liste de l'Écho du Huit",
+    "default_icon": "icons/icon.svg"
+  },
+  "host_permissions": [
+    "https://atelier-huit.frama.space/*",
+    "https://www.ville-cugnaux.fr/*"
+  ],
+  "icons": { "48": "icons/icon.svg", "96": "icons/icon.svg" }
+}