feat: create book picker module for collections

Add new bookPicker.ts module for multi-select book picker modal:

Alpine.store for global state:
- isOpen: Modal visibility state
- selectedBooks: Set<string> for persistent selection across HTMX swaps
- Methods: open, close, toggleBook, isSelected, loadBooks, clearFilters, submit

Key features:
- Selection persists across filter changes (Alpine.store)
- Multi-select with checkbox state management
- Adds books to collection via POST /api/collections/:id/books
- Trigger collection page reload after successful add
- Clear filters resets form fields (preserves selections)
- Uses HTMX for dynamic book grid updates

Critical SSR-first implementation:
- Alpine.store ensures state survives HTMX DOM swaps
- Checkboxes re-rendered by HTMX maintain state via store
- Selection persists across pagination and filter changes
- No DOM state, all state in Alpine reactive store

Replaces non-functional add books button in collections.

web/src/bookPicker.ts

@@ -0,0 +1,144 @@
+// Book Picker for Collections - multi-select book picker modal
+// CRITICAL: Selection state must persist across HTMX grid updates
+
+import { Alpine } from "./alpine";
+import { showToast } from "./toast";
+
+// Alpine.store for global book picker state
+// Using store ensures state persists across HTMX DOM swaps
+Alpine.store("bookPicker", {
+  isOpen: false,
+  selectedBooks: new Set<string>(),
+
+  open() {
+    this.isOpen = true;
+    this.selectedBooks.clear();
+    this.loadBooks();
+  },
+
+  close() {
+    this.isOpen = false;
+    this.selectedBooks.clear();
+  },
+
+  toggleBook(bookId: string) {
+    if (this.selectedBooks.has(bookId)) {
+      this.selectedBooks.delete(bookId);
+    } else {
+      this.selectedBooks.add(bookId);
+    }
+    // Trigger Alpine reactivity by creating new Set
+    this.selectedBooks = new Set(this.selectedBooks);
+  },
+
+  isSelected(bookId: string): boolean {
+    return this.selectedBooks.has(bookId);
+  },
+
+  get selectedCount(): number {
+    return this.selectedBooks.size;
+  },
+
+  async loadBooks() {
+    const grid = document.getElementById("book-picker-grid");
+    if (!grid) return;
+
+    // Trigger initial HTMX load
+    window.htmx.trigger(grid, "loadBooks");
+  },
+
+  clearFilters() {
+    const filterForm = document.getElementById(
+      "book-picker-filters",
+    ) as HTMLFormElement;
+    if (!filterForm) return;
+
+    // Reset all form fields except checkbox state (that's in Alpine.store)
+    const inputs = filterForm.querySelectorAll("input:not([type='checkbox'])");
+    inputs.forEach((input) => {
+      (input as HTMLInputElement).value = "";
+    });
+
+    // Reload books - selection state preserved in Alpine.store
+    this.loadBooks();
+  },
+
+  async submit() {
+    const token = localStorage.getItem("token");
+    if (!token) {
+      showToast("Not authenticated", "error");
+      return;
+    }
+
+    if (this.selectedBooks.size === 0) {
+      showToast("No books selected", "error");
+      return;
+    }
+
+    // Get collection ID from current page
+    const pathParts = window.location.pathname.split("/");
+    const collectionId = pathParts[pathParts.length - 1];
+
+    try {
+      const response = await fetch(`/api/collections/${collectionId}/books`, {
+        method: "POST",
+        headers: {
+          "Content-Type": "application/json",
+          Authorization: `Bearer ${token}`,
+        },
+        body: JSON.stringify({
+          book_ids: Array.from(this.selectedBooks),
+        }),
+      });
+
+      if (response.ok) {
+        showToast(
+          `Added ${this.selectedBooks.size} books to collection`,
+          "success",
+        );
+        this.close();
+        // Reload collection detail page
+        window.htmx.trigger(document.body, "reloadCollection");
+      } else {
+        showToast("Failed to add books", "error");
+      }
+    } catch (error) {
+      console.error("Failed to add books:", error);
+      showToast("Error adding books", "error");
+    }
+  },
+});
+
+// Alpine component for book picker
+// Provides methods for template to access store
+Alpine.data("bookPicker", () => ({
+  get isOpen() {
+    return (window as any).Alpine?.$store.bookPicker.isOpen;
+  },
+
+  get selectedCount() {
+    return (window as any).Alpine?.$store.bookPicker.selectedCount;
+  },
+
+  toggleBook(bookId: string) {
+    (window as any).Alpine?.$store.bookPicker.toggleBook(bookId);
+  },
+
+  isSelected(bookId: string): boolean {
+    return (window as any).Alpine?.$store.bookPicker.isSelected(bookId);
+  },
+
+  clearFilters() {
+    (window as any).Alpine?.$store.bookPicker.clearFilters();
+  },
+
+  close() {
+    (window as any).Alpine?.$store.bookPicker.close();
+  },
+
+  submit() {
+    (window as any).Alpine?.$store.bookPicker.submit();
+  },
+}));
+
+export {};