feat(frontend): SEO — sitemap, canonical link, structured data

- robots.txt now also points at a real sitemap.xml (public pages
  only: /, /download, /themes, /showcase) and disallows the
  auth-gated routes (/account, /configs, /link) that have nothing
  useful to a crawler
- canonical <link> on the root page
- SoftwareApplication JSON-LD so Google can render a rich result
  (name, free price, icon) instead of a plain blue link

Claude-Session: https://claude.ai/code/session_01F1M1Jic1wTSn4igUENynmZ

frontend/public/sitemap.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>https://visual.loki-code.dev/</loc>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+  </url>
+  <url>
+    <loc>https://visual.loki-code.dev/download</loc>
+    <changefreq>weekly</changefreq>
+    <priority>0.8</priority>
+  </url>
+  <url>
+    <loc>https://visual.loki-code.dev/themes</loc>
+    <changefreq>monthly</changefreq>
+    <priority>0.6</priority>
+  </url>
+  <url>
+    <loc>https://visual.loki-code.dev/showcase</loc>
+    <changefreq>daily</changefreq>
+    <priority>0.6</priority>
+  </url>
+</urlset>