Reciprocal hreflang in Astro, only when both languages exist

Astro 5's i18n routing doesn't emit <link rel="alternate" hreflang> for you. To fix this, create helper functions to detect which languages have an entry, and then emit <link rel="alternate"> conditionally in Seo.astro. This prevents reciprocal hreflang mismatches and strengthens the site's multilingual signal.