feat(frontend): add per-language SEO metadata, JSON-LD, robots.txt, and sitemap

Part of the Ombrora Convert redesign (Task 4/5).

frontend/src/pages/HomePage.jsx

@@ -1,4 +1,5 @@
 import { useState } from 'react';
+import { useParams } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { fetchFormats, uploadFiles } from '../api.js';
 import { FileCard } from '../FileCard.jsx';
@@ -6,6 +7,7 @@ import { Dropzone } from '../components/Dropzone.jsx';
 import { FileConfigCard } from '../components/FileConfigCard.jsx';
 import { ReassuranceStrip } from '../components/ReassuranceStrip.jsx';
 import { FormatsGrid } from '../components/FormatsGrid.jsx';
+import { SeoHead } from '../components/SeoHead.jsx';
 import '../styles/home.css';
 import '../styles/sections.css';
 
@@ -29,6 +31,7 @@ function defaultQualityFor(targetFormat) {
 }
 
 export function HomePage() {
+  const { lang } = useParams();
   const { t } = useTranslation();
   const [pendingFiles, setPendingFiles] = useState([]);
   const [submittedJobs, setSubmittedJobs] = useState([]);
@@ -83,6 +86,7 @@ export function HomePage() {
 
   return (
     <>
+      <SeoHead lang={lang} />
       <section className="hero">
         <h1>{t('hero.title')}</h1>
         <p>{t('hero.subtitle')}</p>