feat: SEO crawlability + on-brand copy

Make every served byte crawlable without executing JS, remove off-brand
copy, and cut third-party requests.

- Copy: replace off-brand terms (survival/crisis/collapse) in title, meta,
  OG/Twitter, JSON-LD and webmanifest with on-brand preparedness copy.
  Update App.vue i18n page_title/meta_description (EN+DE) too, since the app
  overwrites the head at runtime. Reconcile area count to the real 6.
- Prerender: vite-plugin-seo-snapshot injects a static <noscript> snapshot
  (real <h1>, content, <a href>) after #app. JS users ignore <noscript>, so
  the live app/UX is unchanged; curl/no-JS crawlers get real content.
- 404: nginx now returns true 404s on unknown paths (no soft-200 SPA
  fallback) with a branded 404 page.
- Sitemap: drop no-op hreflang alternates, add <lastmod>.
- Fonts: self-host Space Mono / DM Serif Display / Barlow (woff2, latin +
  latin-ext, font-display: swap); preload above-the-fold faces. No more
  fonts.googleapis/gstatic requests.

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

public/sitemap.xml

@@ -1,12 +1,9 @@
 <?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">
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
   <url>
     <loc>https://plan-b.now/</loc>
+    <lastmod>2026-06-18</lastmod>
     <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>