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.test.ts

@@ -0,0 +1,45 @@
+import { describe, expect, test } from "bun:test";
+import { existsSync } from "node:fs";
+import { join } from "node:path";
+
+const extensionDir = join(import.meta.dir, "extension");
+const manifest = await Bun.file(join(extensionDir, "manifest.json")).json();
+
+describe("manifest.json", () => {
+  test("est en Manifest V3", () => {
+    expect(manifest.manifest_version).toBe(3);
+  });
+
+  test("déclare background.js comme script d'arrière-plan", () => {
+    expect(manifest.background.scripts).toContain("background.js");
+  });
+
+  test("demande les host_permissions des deux sources", () => {
+    expect(manifest.host_permissions).toEqual([
+      "https://atelier-huit.frama.space/*",
+      "https://www.ville-cugnaux.fr/*",
+    ]);
+  });
+
+  test("ne définit PAS default_popup (sinon onClicked devient muet)", () => {
+    expect(manifest.action.default_popup).toBeUndefined();
+  });
+
+  test("porte un identifiant gecko", () => {
+    expect(manifest.browser_specific_settings.gecko.id).toBeDefined();
+  });
+});
+
+describe("ressources référencées", () => {
+  test("background.js existe", () => {
+    expect(existsSync(join(extensionDir, "background.js"))).toBe(true);
+  });
+
+  test("liste.html existe", () => {
+    expect(existsSync(join(extensionDir, "liste.html"))).toBe(true);
+  });
+
+  test("l'icône référencée existe", () => {
+    expect(existsSync(join(extensionDir, manifest.action.default_icon))).toBe(true);
+  });
+});