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/main.jsx

@@ -1,10 +1,14 @@
-import { StrictMode } from 'react'
-import { createRoot } from 'react-dom/client'
-import './index.css'
-import App from './App.jsx'
+import { StrictMode } from 'react';
+import { createRoot } from 'react-dom/client';
+import { BrowserRouter } from 'react-router-dom';
+import './i18n.js';
+import './index.css';
+import App from './App.jsx';
 
 createRoot(document.getElementById('root')).render(
   <StrictMode>
-    <App />
-  </StrictMode>,
-)
+    <BrowserRouter>
+      <App />
+    </BrowserRouter>
+  </StrictMode>
+);