docs(meta): sharpen the base SEO description

Replace the generic page description with the self-hosted positioning —
Withings + Apple Health sync, transparent derived wellness metrics, and
AI Insights you own — so the search-result snippet matches the README
and landing copy. OpenGraph and Twitter card descriptions are unchanged.

src/app/layout.tsx

@@ -35,7 +35,7 @@ const inter = Inter({
 export const metadata: Metadata = {
   title: "HealthLog",
   description:
-    "Personal health tracking — weight, blood pressure, pulse, mood, medication compliance",
+    "Self-hosted health tracker — weight, blood pressure, glucose, mood, medications. Withings + Apple Health sync, transparent derived wellness metrics, AI Insights you own.",
   manifest: "/manifest.json",
   icons: {
     icon: "/favicon.svg",