feat(library-switcher): add shared library switcher component and module

Add reusable library switcher infrastructure that can be used across
dashboard, collections list, and collection detail pages.

New files:
- templates/library_switcher.templ: Shared LibrarySwitcher component
  with variadic actions slot for page-specific buttons (e.g. dashboard
  settings/refresh). Includes DashboardActions sub-component.
- web/src/library-switcher.ts: Shared module providing:
  - initLibrarySwitcher(): syncs dropdown with localStorage, attaches
    change listener with configurable onSwitch callback
  - switchWithTransition(): generic fade-out -> spinner -> fetch ->
    fade-in transition used by all pages
  - getCurrentLibraryId(): reads "selectedLibrary" from localStorage

Modified:
- web/src/main.ts: import new library-switcher module
- web/src/types/api.d.ts: add book_count field to CollectionData

web/src/library-switcher.ts

@@ -0,0 +1,83 @@
+import { Alpine } from "./alpine";
+import { showToast } from "./toast";
+
+const STORAGE_KEY = "selectedLibrary";
+
+export function getCurrentLibraryId(): string {
+  return localStorage.getItem(STORAGE_KEY) || "";
+}
+
+export async function switchWithTransition(
+  containerId: string,
+  fetchFn: () => Promise<void>,
+): Promise<void> {
+  const container = document.getElementById(containerId);
+  const loading = document.getElementById("loading-spinner");
+
+  if (!container || !loading) return;
+
+  try {
+    container.classList.add(
+      "opacity-0",
+      "transition-opacity",
+      "duration-150",
+    );
+    await new Promise((resolve) => setTimeout(resolve, 150));
+
+    loading.classList.remove("hidden");
+
+    await fetchFn();
+
+    container.classList.remove("duration-150");
+    container.classList.add("duration-300");
+    void container.offsetHeight;
+    container.classList.remove("opacity-0");
+
+    setTimeout(() => {
+      container.classList.remove("transition-opacity", "duration-300");
+    }, 300);
+  } catch (error) {
+    container.classList.remove(
+      "opacity-0",
+      "transition-opacity",
+      "duration-150",
+      "duration-300",
+    );
+    showToast("Failed to load library", "error");
+    console.error("Switch library error:", error);
+  } finally {
+    loading.classList.add("hidden");
+  }
+}
+
+export interface LibrarySwitcherOptions {
+  onSwitch: (libraryId: string) => Promise<void>;
+}
+
+export function initLibrarySwitcher(options: LibrarySwitcherOptions): void {
+  const librarySelect = document.getElementById(
+    "library-select",
+  ) as HTMLSelectElement;
+  if (!librarySelect) return;
+
+  const stored = localStorage.getItem(STORAGE_KEY);
+  if (stored) {
+    const option = librarySelect.querySelector(
+      `option[value="${stored}"]`,
+    );
+    if (option) {
+      librarySelect.value = stored;
+    }
+  }
+
+  librarySelect.addEventListener("change", async (e) => {
+    const target = e.target as HTMLSelectElement;
+    if (!target.value && target.value !== "") return;
+
+    const libraryId = target.value;
+    localStorage.setItem(STORAGE_KEY, libraryId);
+    await options.onSwitch(libraryId);
+  });
+}
+
+Alpine.data("librarySwitcher", () => ({}));

web/src/main.ts

@@ -21,6 +21,7 @@ import "./events";
 import "./header";
 import "./index";
 import "./library";
+import "./library-switcher";
 import "./linking";
 import "./login";
 import "./password_validation";

web/src/types/api.d.ts

@@ -90,6 +90,7 @@ interface CollectionData {
   icon: string;
   auto_assign_rules?: unknown;
   created_at: string;
+  book_count?: number;
 }
 
 // Matches handlers.BookInfo JSON response (internal/handlers/collections.go:66-71)