feat: add an embed mode that shows only the globe

?embed=1 shows the globe alone: clean view with the HUD, panels, welcome and
setup prompts hidden, and provider attribution kept. The page that frames it
sends views with postMessage; the app applies each through its own actions
and reports every step. Annotations in any link are drawn once it restores.
Only embed-mode documents may be framed; every other page keeps
X-Frame-Options DENY and frame-ancestors 'none', and
GEV_EMBED_FRAME_ANCESTORS can restrict who frames them.

src/firstRunExperience.js

@@ -211,6 +211,8 @@ export function shouldShowFirstRun({
   if (hasShareState) return false;
   const params = new URLSearchParams(location?.search || '');
   if (params.get('welcome') === '0') return false;
+  // An embedded globe never shows the welcome launcher.
+  if (params.get('embed') === '1') return false;
   // The demo/support escape hatch outranks both suppressions on purpose.
   if (params.get('welcome') === '1') return true;
   if (readStored('local', storage, FIRST_RUN_STORAGE_KEY) === 'suppressed')