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">

web/src/bookshelf.ts

@@ -415,6 +415,9 @@ function initBookshelf(): void {
     const librarySelect = document.getElementById(
       "library-select",
     ) as HTMLSelectElement;
+    librarySelect?.addEventListener("change", () => {
+      localStorage.setItem("selectedLibrary", librarySelect.value);
+    });
     if (librarySelect && librarySelect.value) {
       const filterForm = document.getElementById(
         "filter-form",

web/src/header.ts

@@ -9,6 +9,7 @@ import {
   loadWoodPaneling,
   updateWoodPanelingIndicators,
 } from "./theme";
+import { getSelectedLibrary } from "./storage";
 
 const logout = (): void => {
   localStorage.removeItem("token");
@@ -16,7 +17,34 @@ const logout = (): void => {
   window.location.href = "/";
 };
 
-export { logout };
+// Looks for saved library in localStorage and selects it in current page's dropdown
+const restoreLibrarySelection = (): void => {
+  const savedLibrary = getSelectedLibrary();
+  if (!savedLibrary) return;
+
+  // Common library dropdown IDs across pages
+  const dropdownSelectors = [
+    "#library-select", // dashboard, bookshelf
+    "#selected-library", // alternate naming
+    "#library_id", // form field
+  ];
+
+  for (const selector of dropdownSelectors) {
+    const dropdown = document.querySelector(selector) as HTMLSelectElement;
+    if (dropdown) {
+      const option = dropdown.querySelector(`option[value="${savedLibrary}"]`);
+      if (option) {
+        dropdown.value = savedLibrary;
+        // Trigger any htmx/change handlers
+        dropdown.dispatchEvent(new Event("change", { bubbles: true }));
+        console.log("Restored library selection:", savedLibrary);
+        return;
+      }
+    }
+  }
+};
+
+export { logout, restoreLibrarySelection };
 
 Alpine.data("header", () => ({
   logout,
@@ -26,4 +54,5 @@ Alpine.data("header", () => ({
   changeWoodPaneling,
   loadWoodPaneling,
   updateWoodPanelingIndicators,
+  restoreLibrarySelection,
 }));