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

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",