refactor: remove unnecessary DOMContentLoaded wrappers

Since main.js has 'defer' attribute, the DOM is guaranteed to be
ready when modules execute. These wrappers are unnecessary.

dashboard.ts:
- Removed DOMContentLoaded wrapper, code runs directly
- Event delegation setup runs immediately

custom-section-builder.ts:
- Removed DOMContentLoaded wrapper
- initCustomSectionBuilder() called directly

toast.ts:
- Removed DOMContentLoaded wrapper
- initializeToastSystem() called directly at top level
- Removed dead Alpine.data registration (unused)

search.ts:
- Removed DOMContentLoaded wrapper
- initializeSearch exported for use in header

theme.ts:
- Removed DOMContentLoaded wrapper
- Functions now exported for use in header Alpine component

web/src/custom-section-builder.ts

@@ -651,6 +651,6 @@ function builderEscapeHtml(text: string): string {
   return div.innerHTML;
 }
 
-document.addEventListener("DOMContentLoaded", initCustomSectionBuilder);
+initCustomSectionBuilder();
 
 export { addBookToSelection, removeBookFromSelection };