fix: decouple Head.tsx from og-image plugin import (#2495)

Remove unconditional import of CustomOgImagesEmitterName from
.quartz/plugins in Head.tsx. This caused builds to fail when the
og-image plugin was not listed in quartz.config.yaml, violating the
plugin system's opt-in contract.

The fix inlines the emitter name string constant at the usage site.

Also convert the ContentDetails import in fileTrie.ts to
`import type` since it is only used in a type position.

quartz/util/fileTrie.ts

@@ -1,4 +1,4 @@
-import { ContentDetails } from "../../.quartz/plugins"
+import type { ContentDetails } from "../../.quartz/plugins"
 import { FullSlug, joinSegments } from "./path"
 
 interface FileTrieData {