refactor(header): integrate search and theme functions via Alpine

- header.ts now imports and re-exports functions from search.ts
  and theme.ts for use in the header template
- Functions available via x-data=header:
  - initializeSearch
  - initializeTheme
  - changeTheme
  - changeWoodPaneling
  - loadWoodPaneling
  - updateWoodPanelingIndicators
- header.templ x-init calls these functions directly
- Enables proper SSR-first pattern with x-init for setup only

templates/header.templ

@@ -1,7 +1,7 @@
 package templates
 
 templ Header(user User, currentPath string) {
-	<nav x-data="theme" class="border-b header-nav" style="border-color: var(--border); background-color: var(--bg-secondary);">
+	<nav x-data="header" x-init="initializeSearch(); initializeTheme(); loadWoodPaneling(); updateWoodPanelingIndicators()" class="border-b header-nav" style="border-color: var(--border); background-color: var(--bg-secondary);">
 		<div x-data="{}" x-init="console.log('Alpine is working!', $el)"></div>
 		<div class="w-full px-4 sm:px-6 lg:px-8">
 			<div class="flex justify-between items-center h-16">