feat(frontend): add routing, layout, i18n wiring, and restyle the conversion tool

Migrates the existing upload/format-select/quality/convert logic from
the old single-file App.jsx into a router shell (/ -> /fr//en/) with a
shared Layout (header, language switcher, dark-mode toggle, footer).
Tool logic itself is unchanged, only restyled into cards.

Part of the Ombrora Convert redesign (Task 2/5).

frontend/src/FileCard.jsx

@@ -1,7 +1,10 @@
 import { useEffect, useState } from 'react';
+import { useTranslation } from 'react-i18next';
+import { DownloadSimple } from '@phosphor-icons/react';
 import { fetchJobStatus, downloadUrl } from './api.js';
 
 export function FileCard({ fileName, jobId, initialError }) {
+  const { t } = useTranslation();
   const [status, setStatus] = useState(initialError ? 'failed' : 'pending');
   const [errorMessage, setErrorMessage] = useState(initialError ?? null);
 
@@ -25,9 +28,13 @@ export function FileCard({ fileName, jobId, initialError }) {
 
   return (
     <li className="file-card">
-      <span className="file-name">{fileName}</span>
-      {(status === 'pending' || status === 'processing') && <span>Conversion en cours...</span>}
-      {status === 'done' && <a href={downloadUrl(jobId)}>Télécharger</a>}
+      <span className="file-config-name">{fileName}</span>
+      {(status === 'pending' || status === 'processing') && <span>{t('job.converting')}</span>}
+      {status === 'done' && (
+        <a href={downloadUrl(jobId)}>
+          <DownloadSimple size={20} weight="regular" /> {t('job.download')}
+        </a>
+      )}
       {status === 'failed' && <span className="error">{errorMessage}</span>}
     </li>
   );