feat(mdbook): version-picker always renders a label

Previously the picker silently hid when the URL didn't contain a
/<version>/ segment — which is exactly the case for `just build`
opened via file://. The reader saw no version indicator at all.

Now the picker always mounts a "version: latest" label first
(immediately, before any fetch). When a versions.json is reachable
AND we have a real version segment in the URL (so navigation is
computable), the label upgrades into the existing dropdown. On
file:// or single-version sites the static label stays.

CSS gains styling for the static "version: <name>" name span.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

theme/version-picker.css

@@ -30,6 +30,14 @@
   outline: none;
 }
 
+.version-picker-name {
+  color: var(--fg);
+  font-weight: 600;
+  padding: 0.2em 0.45em;
+  border: 1px solid transparent;
+  border-radius: 3px;
+}
+
 .version-picker-floating {
   position: fixed;
   top: 0.5em;