fix(landing): fetch launcher downloads from live launcher.json on load

The build-time download buttons baked stale/placeholder URLs (the host has no
synced launcher.json, so the build fell back to launcher.example.json). Render
a skeleton at build time instead and fetch the live launcher.json client-side
once on page load (no polling) from
https://distribution.${BASE_DOMAIN}/launcher/launcher.json, building per-OS
buttons from files[]. On fetch error the skeleton clears.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

landing/src/styles/main.css

@@ -207,6 +207,26 @@ section { position: relative; z-index: 1; }
 }
 .mc-btn.sm { font-size: 14px; padding: 10px 16px 12px; }
 
+/* Launcher download skeleton (placeholder while launcher.json loads). */
+.dl-skeleton {
+  min-width: 9.5rem;
+  color: transparent;
+  pointer-events: none;
+  background-image: linear-gradient(
+    90deg,
+    oklch(1 0 0 / 0.04),
+    oklch(1 0 0 / 0.14),
+    oklch(1 0 0 / 0.04)
+  );
+  background-size: 200% 100%;
+  animation: dl-shimmer 1.2s linear infinite;
+}
+.dl-skeleton::after { content: "\00a0"; } /* keep height when text is empty */
+@keyframes dl-shimmer {
+  from { background-position: 200% 0; }
+  to { background-position: -200% 0; }
+}
+
 /* ---- Copy-IP chip ---- */
 .ip-chip {
   display: inline-flex;