feat(ui): persist library selection across page navigation

Save the selected library to localStorage when the user changes the
dropdown on the bookshelf page, and restore it on every page load via
a new restoreLibrarySelection() call in the header Alpine component.

This ensures that when a user navigates between dashboard, bookshelf,
collections, etc., their last-chosen library filter is automatically
re-applied rather than resetting to the default.

Changes:
- web/src/bookshelf.ts: listen for change events on #library-select
  and persist the value to localStorage
- web/src/header.ts: add restoreLibrarySelection() which checks
  localStorage and sets the matching dropdown option on page load
- templates/header.templ: call restoreLibrarySelection() in x-init
- templates/header_templ.go: regenerated from templ source

templates/header.templ

@@ -1,7 +1,7 @@
 package templates
 
 templ Header(user User, currentPath string) {
-	<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);">
+	<nav x-data="header" x-init="initializeSearch(); initializeTheme(); loadWoodPaneling(); updateWoodPanelingIndicators(); restoreLibrarySelection()" 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">