fix: resolve client-side navigation and popover issues

- Fix popover crash on missing Content-Type header (null check)
- Fix SPA loading bar corrupting page via micromorph interference
  (remove bar before morph, use ephemeral DOM elements instead of
  persistent reference)
- Add stopLoading() cleanup in navigation finally block

quartz/components/scripts/popover.inline.ts

@@ -60,7 +60,9 @@ async function mouseEnterHandler(
   })
 
   if (!response) return
-  const [contentType] = response.headers.get("Content-Type")!.split(";")
+  const rawContentType = response.headers.get("Content-Type")
+  if (!rawContentType) return
+  const [contentType] = rawContentType.split(";")
   const [contentTypeCategory, typeInfo] = contentType.split("/")
 
   const popoverElement = document.createElement("div")