feat: implement saved filter loading via API endpoint

Implement Phase 8 of GET_SAVED_FILTER_BY_ID_IMPLEMENTATION.md:
Frontend integration for loading saved filters via GET /:id endpoint.
Completes the saved filters feature with full CRUD functionality.

Changes to web/src/bookshelf.ts:
- Convert loadFilter() from synchronous to async function
- Fetch filter details from GET /api/saved-filters/:id endpoint
- Parse filters JSON (handles both string and object formats)
- Populate hidden #filter-form fields with filter values
- Update visible form fields for user feedback
- Trigger HTMX change event to apply filter
- Show loading, success, and error toasts
- Close filters dropdown after applying filter
- Proper error handling (404, network errors, auth errors)

User Flow:
1. User clicks saved filter in dropdown (server-rendered list)
2. Alpine.js calls GET /api/saved-filters/:id API
3. Receives filter object with filters JSONB
4. Populates form fields (hidden + visible)
5. Triggers HTMX to submit form
6. Books grid updates instantly (no page reload)

SSR-First Compliance:
- ✅ Initial page load: Server renders everything (no API calls)
- ✅ User interaction only: API called when user clicks filter
- ✅ Hybrid approach: Alpine fetches data, HTMX applies it
- ✅ No async x-init data fetching
- ✅ Progressive enhancement maintained
- ✅ Matches dashboard pattern for interactions

Error Handling:
- 404: Filter not found (deleted by another session)
- 401: Not authenticated
- Network errors: Show error toast
- Form not found: Show error toast

Benefits:
- Complete saved filters CRUD functionality
- Instant filter application (no page reload)
- User feedback with toast notifications
- Follows HTMX + Alpine hybrid pattern
- Type-safe TypeScript with proper error handling

Implements Phase 8 from GET_SAVED_FILTER_BY_ID_IMPLEMENTATION.md.

web/src/bookshelf.ts

@@ -41,7 +41,7 @@ Alpine.data("bookshelf", () => ({
     this.showFiltersDropdown = !this.showFiltersDropdown;
   },
   // Load a saved filter into the form
-  loadFilter(event: Event) {
+  async loadFilter(event: Event) {
     const button = event.target as HTMLElement;
     const filterRow = button.closest("[data-filter-id]");
     if (!filterRow) return;
@@ -51,9 +51,87 @@ Alpine.data("bookshelf", () => ({
 
     const filterName = button.textContent?.trim() || "";
 
-    // Note: Actual filter loading logic would go here
+    showToast(`Loading filter: ${filterName}...`, "info");
-    // For now, just show the filter name
+
-    showToast(`Filter: ${filterName}`, "success");
+    const token = localStorage.getItem("token");
+    if (!token) {
+      showToast("Not authenticated", "error");
+      return;
+    }
+
+    try {
+      // Fetch filter details from API
+      const response = await fetch(`/api/saved-filters/${filterId}`, {
+        headers: { Authorization: `Bearer ${token}` },
+      });
+
+      if (!response.ok) {
+        if (response.status === 404) {
+          showToast("Filter not found", "error");
+        } else {
+          showToast("Failed to load filter", "error");
+        }
+        return;
+      }
+
+      const filter = await response.json();
+
+      // Parse filters JSON (string → object)
+      const filterData: Record<string, string> =
+        typeof filter.filters === "string"
+          ? JSON.parse(filter.filters)
+          : filter.filters;
+
+      // Get the hidden filter form
+      const filterForm = document.getElementById(
+        "filter-form",
+      ) as HTMLFormElement;
+      if (!filterForm) {
+        showToast("Filter form not found", "error");
+        return;
+      }
+
+      // Clear existing filter values
+      filterForm.innerHTML = `
+          <input type="hidden" name="limit" value="50"/>
+          <input type="hidden" name="offset" value="0"/>
+      `;
+
+      // Populate form fields from filter data
+      Object.entries(filterData).forEach(([key, value]) => {
+        if (value) {
+          // Only set non-empty values
+          const input = document.createElement("input");
+          input.type = "hidden";
+          input.name = key;
+          input.value = value;
+          filterForm.appendChild(input);
+
+          // Also update visible form fields if they exist
+          const visibleField = document.querySelector(
+            `[name="${key}"]`,
+          ) as HTMLInputElement;
+          if (visibleField) {
+            visibleField.value = value;
+          }
+        }
+      });
+
+      // Trigger HTMX to apply the filter
+      // Use the first input to trigger the change event
+      const firstInput = filterForm.querySelector("input");
+      if (firstInput) {
+        window.htmx.trigger(firstInput, "change");
+      }
+
+      showToast(`Filter applied: ${filterName}`, "success");
+
+      // Close the dropdown after applying
+      this.showFiltersDropdown = false;
+    } catch (error) {
+      console.error("Failed to load filter:", error);
+      showToast("Error loading filter", "error");
+    }
   },
   // Delete a saved filter
   async deleteFilter(event: Event) {
@@ -155,13 +233,18 @@ Alpine.data("bookshelf", () => ({
 function initBookshelf(): void {
   // Check if books were already rendered server-side
   const booksGrid = document.getElementById("books-grid");
-  const hasServerBooks = booksGrid && booksGrid.querySelector('[data-book-id]') !== null;
+  const hasServerBooks =
+    booksGrid && booksGrid.querySelector("[data-book-id]") !== null;
 
   if (!hasServerBooks) {
     // Only trigger HTMX if no SSR books rendered
-    const librarySelect = document.getElementById("library-select") as HTMLSelectElement;
+    const librarySelect = document.getElementById(
+      "library-select",
+    ) as HTMLSelectElement;
     if (librarySelect && librarySelect.value) {
-      const filterForm = document.getElementById("filter-form") as HTMLFormElement;
+      const filterForm = document.getElementById(
+        "filter-form",
+      ) as HTMLFormElement;
       if (filterForm) {
         // Trigger initial HTMX load
         window.htmx.trigger(librarySelect, "change");