fix(maps): narrow the PDF map container and assert the 3D toast by its key

The offscreen container is built inside buildGlMap's closure, where the
let lost its narrowing; a const of its own keeps it an HTMLDivElement.
The stage tests hand the stage a t that returns the key, so the toast
reads map.webgl2.terrain there. One floating promise fewer lowers the
eslint baseline to 164.

client/scripts/eslint-baseline.json

@@ -10,7 +10,7 @@
     "trek/store-without-selector": 67,
     "react-hooks/refs": 555,
     "react-hooks/set-state-in-effect": 194,
-    "@typescript-eslint/no-floating-promises": 165,
+    "@typescript-eslint/no-floating-promises": 164,
     "react-hooks/exhaustive-deps": 116,
     "@typescript-eslint/no-unused-vars": 124,
     "react-hooks/immutability": 28,