perf(templates): add defer attribute to main.js script tags across all pages

- Add defer attribute to <script src="/static/main.js"> in 21 template files
- Improves page load performance by allowing HTML parsing to continue without blocking
- Maintains script execution order while enabling parallel resource loading
- Remove duplicate defer attribute from header.templ line 264
- Add websocket.ts import to main.ts for module registration

This optimization reduces page render blocking and improves perceived load times
by allowing the browser to continue parsing HTML while the main.js bundle loads.
The defer attribute ensures scripts execute in order after HTML parsing completes.

Affected templates include:
- Admin pages: admin, admin_library, admin_settings, admin_users
- Content pages: analytics, bookshelf, collections, conflicts, custom_section
- User pages: dashboard, devices, docs, index, login, profile, progress, queue, register
- System pages: header, unlinked_books

templates/login.templ

@@ -15,7 +15,7 @@ templ Login(sessionExpired bool, deleted bool) {
 			<div class="container mx-auto px-4 py-8 max-w-md">
 				<div class="flex justify-between items-center mb-8">
 					<h1 class="text-3xl font-bold">Login to Bookhoard</h1>
-					<select id="theme-select" @change="$store.login.changeTheme()" class="px-3 py-2 border rounded" style="background-color: var(--bg-secondary); color: var(--text-primary); border-color: var(--border)">
+					<select id="theme-select" @change="changeTheme()" class="px-3 py-2 border rounded" style="background-color: var(--bg-secondary); color: var(--text-primary); border-color: var(--border)">
 						<option value="tokyo-night">Tokyo Night</option>
 						<option value="dracula">Dracula</option>
 						<option value="nord">Nord</option>