chore(frontend/test): normalize test environement

normalize environment to avoid such issues during tests
    on non EN local machine:

    ```
    AssertionError: expected 'il y a 2 ans' to match /year/

   ❯ src/lib/utils/time.test.ts:24:60
       22|
       23|  it('formats past times in the largest matching unit', () => {
       24|   expect(relativeTimeAgo(Date.now() - 2 * 31_536_000_000)).toMatch(/year/);
    ```

frontend/package.json

@@ -14,9 +14,9 @@
     "check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",
     "lint": "eslint .",
     "format": "prettier --write .",
-    "test:unit": "vitest run",
-    "test:unit:watch": "vitest",
-    "test:unit:coverage": "rm -rf ../tests/e2e/.nyc_output_unit && COVERAGE=1 vitest run"
+    "test:unit": "LANG=C vitest run",
+    "test:unit:watch": "LANG=C vitest",
+    "test:unit:coverage": "rm -rf ../tests/e2e/.nyc_output_unit && LANG=C COVERAGE=1 vitest run"
   },
   "devDependencies": {
     "@eslint/js": "^10.0.1",