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/robots.txt

@@ -1,2 +1,8 @@
 User-agent: *
 Allow: /
+# Requires a signed-in account; nothing here is useful to crawl.
+Disallow: /account
+Disallow: /configs
+Disallow: /link
+
+Sitemap: https://visual.loki-code.dev/sitemap.xml