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/images/favicon.svg

@@ -0,0 +1,7 @@
+<svg width="96" height="96" viewBox="0 0 96 96" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="Plan-B">
+  <circle cx="48" cy="48" r="44" fill="#2a3010"/>
+  <g transform="translate(19.75 21.15) scale(0.46)" fill="#f4ecd8">
+    <path d="M44.2,71.76 c2.03,0,3.68,1.65,3.68,3.68c0,2.03-1.65,3.68-3.68,3.68H23.38l-0.46-0.04c-2.67-0.34-5.09-0.97-7.29-1.88 c-2.25-0.93-4.26-2.14-6.04-3.63H9.58c-1.68-1.4-3.15-2.99-4.4-4.72C1.84,64.25,0.04,58.63,0,53.03 c-0.04-5.66,1.72-11.29,5.52-15.85c1.23-1.48,2.68-2.84,4.34-4.04c1.93-1.4,4.14-2.58,6.64-3.55c1.72-0.67,3.56-1.23,5.5-1.68 c2.2-8.74,6.89-15.47,12.92-20.14c5.64-4.37,12.43-6.92,19.42-7.59c6.96-0.67,14.12,0.51,20.55,3.6 c7.37,3.54,13.43,9.56,17.11,16.87c1.6-0.25,3.2-0.38,4.79-0.36c6.72,0.05,13.2,2.45,18.3,7.95c5.31,5.72,7.88,14.14,7.79,21.82 c-0.07,6.31-1.77,12.59-5.25,17.22c-2.27,3.02-5.18,5.47-8.67,7.42c-3.36,1.88-7.28,3.31-11.68,4.33c-1.98,0.45-3.95-0.78-4.4-2.76 c-0.45-1.98,0.78-3.95,2.76-4.4c3.71-0.86,6.97-2.04,9.72-3.58c2.63-1.47,4.78-3.26,6.39-5.41c2.5-3.33,3.73-8.04,3.78-12.87 c0.06-5.07-1.18-10.16-3.59-13.86c-0.69-1.07-1.44-2.03-2.25-2.89c-3.61-3.89-8.19-5.59-12.95-5.62 c-3.46-0.02-7.02,0.81-10.41,2.31c-0.75,0.37-1.51,0.78-2.25,1.21c-2.25,1.32-4.48,2.93-6.74,4.78l-4.84-5.54 c1.67-1.55,3.48-2.96,5.4-4.21c1.53-1,3.13-1.88,4.77-2.65c0.66-0.33,1.33-0.64,2-0.93c-3.19-5.65-7.78-9.7-12.98-12.2 c-5.2-2.49-11.02-3.45-16.69-2.9c-5.63,0.54-11.1,2.59-15.62,6.1c-5.23,4.06-9.2,10.11-10.73,18.14l-0.48,2.51l-2.5,0.44 c-2.45,0.43-4.64,1.02-6.56,1.77c-1.86,0.72-3.52,1.61-4.97,2.66c-1.16,0.84-2.16,1.78-3.01,2.8c-2.63,3.15-3.85,7.1-3.82,11.1 c0.03,4.06,1.35,8.16,3.79,11.53c0.91,1.25,1.96,2.4,3.16,3.4l-0.01,0.01c1.2,1,2.58,1.83,4.13,2.47c1.53,0.63,3.22,1.08,5.09,1.34 H44.2L44.2,71.76z"/>
+  </g>
+  <path d="M47.2 49.5 L57.6 49.5 L54 56.5 L60.2 56.6 L45.2 74.9 L48.1 62.9 L43.2 62.9 Z" fill="#fff8dc"/>
+</svg>

public/images/og-card.svg

@@ -0,0 +1,42 @@
+<svg width="1200" height="630" viewBox="0 0 1200 630" xmlns="http://www.w3.org/2000/svg">
+  <defs>
+    <!-- Glossy paint sheen on the wordmark, matching the hero's
+         #2a2a26 → #1A1A18 → #060604 vertical gradient. -->
+    <linearGradient id="paint" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#2a2a26"/>
+      <stop offset="0.45" stop-color="#1A1A18"/>
+      <stop offset="1" stop-color="#060604"/>
+    </linearGradient>
+    <filter id="ds" x="-20%" y="-20%" width="140%" height="160%">
+      <feGaussianBlur in="SourceAlpha" stdDeviation="6"/>
+      <feOffset dx="0" dy="7" result="o"/>
+      <feComponentTransfer in="o" result="s"><feFuncA type="linear" slope="0.28"/></feComponentTransfer>
+      <feMerge><feMergeNode in="s"/><feMergeNode in="SourceGraphic"/></feMerge>
+    </filter>
+  </defs>
+
+  <!-- Flat hero background -->
+  <rect width="1200" height="630" fill="#F0F0F0"/>
+
+  <!-- Embossed multilingual "Plan B" pattern (the homepage page-bg),
+       tilted ~-5deg and held very faint behind the wordmark. -->
+  <g transform="rotate(-5 600 315)" fill="#1A1A18" opacity="0.04"
+     font-family="Georgia, 'Times New Roman', serif" font-style="italic" font-weight="700" font-size="64" letter-spacing="6">
+    <text x="-60" y="70">Plan B&#8194;Piano B&#8194;Plano B&#8194;План Б&#8194;プランB&#8194;Plan B</text>
+    <text x="-160" y="190">Phương án B&#8194;备用计划&#8194;Plan B&#8194;플랜 B&#8194;Piano B&#8194;योजना बी</text>
+    <text x="-90" y="310">Piano B&#8194;Plan B&#8194;Σχέδιο Β&#8194;Plano B&#8194;خطة ب&#8194;プランB</text>
+    <text x="-200" y="430">Plan B&#8194;תוכנית ב&#8194;プランB&#8194;План Б&#8194;Plan B&#8194;Piano B</text>
+    <text x="-60" y="550">备用计划&#8194;Plan B&#8194;योजना बी&#8194;Plano B&#8194;Plan B&#8194;Σχέδιο Β</text>
+    <text x="-150" y="670">Plan B&#8194;Piano B&#8194;План Б&#8194;Plan B&#8194;プランB&#8194;Plano B</text>
+  </g>
+
+  <!-- Hero wordmark — centred, glossy dark paint, like the homepage. -->
+  <text x="600" y="372" text-anchor="middle" filter="url(#ds)"
+        font-family="Georgia, 'Times New Roman', serif" font-weight="700"
+        font-size="190" letter-spacing="9" fill="url(#paint)">Plan-B</text>
+
+  <!-- Tagline -->
+  <text x="600" y="460" text-anchor="middle"
+        font-family="'Helvetica Neue', Arial, sans-serif" font-weight="500"
+        font-size="23" letter-spacing="7" fill="#0a0a0a">AI-ASSISTED PREPAREDNESS FOR URBAN HOUSEHOLDS</text>
+</svg>

public/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://plan-b.now/sitemap.xml

public/site.webmanifest

@@ -0,0 +1,16 @@
+{
+  "name": "Plan-B — Crisis Preparedness Advisor",
+  "short_name": "Plan-B",
+  "description": "AI-assisted crisis-preparedness advisor for urban households.",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#FAFAFA",
+  "theme_color": "#5A9A78",
+  "lang": "en",
+  "icons": [
+    { "src": "/images/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/images/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    { "src": "/images/favicon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any" }
+  ]
+}

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>