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/ui/styles/embed.css

@@ -0,0 +1,14 @@
+/* Embed mode (?embed=1): the globe alone, on top of clean view. Provider
+   attribution stays visible. */
+body.ui-embed #clean-view-exit,
+body.ui-embed #intel-hud,
+body.ui-embed #key-setup-chip,
+body.ui-embed #key-setup,
+body.ui-embed #first-run-launcher,
+body.ui-embed #command-dock,
+body.ui-embed #left-panel-stack,
+body.ui-embed #right-context-rail,
+body.ui-embed #world-overlay-actions,
+body.ui-embed #view-switcher {
+  display: none !important;
+}