Annotate templates with TODOs for markup replacement

To open source the project down the line, it makes sense to have either
everything custom or using OSS components in order to keep licensing as
simple as possible.

lib/music_library_web/components/layouts/app.html.heex

@@ -1,3 +1,4 @@
+<%!-- TODO: replace with OSS version --%>
 <nav class="border-b border-slate-900/10 dark:border-slate-300/10">
   <div class="mx-auto max-w-screen-xl px-4 sm:px-6 lg:px-8">
     <div class="flex h-16 justify-between">