refactor(templates): consolidate header.js loading

- Move header.js script from individual templates to header.templ
- Removes duplicate script tags from bookshelf, collections, progress, etc.
- Fixes indentation in docs.templ

templates/progress.templ

@@ -11,7 +11,6 @@ templ Progress(user User, progressData []handlers.ProgressWithMedia, errorMessag
         <title>Reading Progress - Bookhoard</title>
         <script src="/static/htmx.min.js"></script>
         <script src="/static/toast.js"></script>
-        <script src="/static/header.js"></script>
         <link href="/static/style.css" rel="stylesheet">
     </head>
     <body class="theme-{ user.Theme }">