feat: add SEO meta, OG image, favicons and sitemap

- Full meta suite in index.html: description, canonical, robots,
  Open Graph + Twitter cards, theme-color and JSON-LD structured data
- 1200x630 OG share card matching the homepage Plan-B wordmark
- Favicon set generated from the intro storm icon (svg + png + apple
  touch + 192/512 PWA icons) and a web manifest
- robots.txt + sitemap.xml with hreflang alternates
- setLang() now syncs title/description/og tags on EN/DE toggle

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

public/sitemap.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
+        xmlns:xhtml="http://www.w3.org/1999/xhtml">
+  <url>
+    <loc>https://plan-b.now/</loc>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+    <xhtml:link rel="alternate" hreflang="en" href="https://plan-b.now/"/>
+    <xhtml:link rel="alternate" hreflang="de" href="https://plan-b.now/"/>
+    <xhtml:link rel="alternate" hreflang="x-default" href="https://plan-b.now/"/>
+  </url>
+</urlset>