fix: coverage JS avec istanbul au lieu de v8 (incompatible Bun)

- vitest.config.js : provider changé de 'v8' à 'istanbul' car v8 utilise
  le Node Inspector API non supporté par Bun
- package.json : ajout @vitest/coverage-istanbul comme devDependency
- Résultat : 17 tests JS, 77% stmts, 63% branches, 75% funcs, 77% lines

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -16,6 +16,7 @@
         "@hotwired/stimulus": "^3.0.0",
         "@spomky-labs/pwa-bundle": "file:vendor/spomky-labs/pwa-bundle/assets",
         "@tailwindcss/postcss": "^4.1.18",
+        "@vitest/coverage-istanbul": "^4.1.2",
         "@vitest/coverage-v8": "^4.1.0",
         "eslint": "9",
         "idb": "^8.0.3",

vitest.config.js

@@ -5,7 +5,7 @@ export default defineConfig({
         environment: 'happy-dom',
         include: ['tests/js/**/*.test.js'],
         coverage: {
-            provider: 'v8',
+            provider: 'istanbul',
             include: ['assets/**/*.js'],
             exclude: ['assets/modules/event-map.js', 'assets/modules/billet-designer.js', 'assets/modules/stripe-payment.js'],
             reporter: ['text', 'lcov'],