feat(frontend): implement library edit functionality

- Reuse Create Library modal for edit mode
- Add hidden library-id input to track create vs edit
- Update handleCreateLibrarySubmit to detect mode and use PUT vs POST
- Implement editLibrary() to populate modal with existing data
- Pass library data to Edit button via data attributes
- Reset modal title when opening for create mode

Fixes: Issue 3

templates/admin_library.templ

@@ -122,6 +122,7 @@ templ AdminLibrary(user User, libraries []LibraryData, users []User) {
                     <button type="button" data-action="hide-create-modal" class="p-2 hover:opacity-80 rounded" style="color: var(--text-primary)">✕</button>
                 </div>
                 <form id="create-library-form">
+                    <input type="hidden" id="library-id" name="id">
                     <div class="mb-4">
                         <label class="block text-sm font-medium mb-2" style="color: var(--text-primary)">Library Name</label>
                         <input type="text" name="name" placeholder="My Ebook Library" class="w-full px-3 py-2 border rounded-lg" style="background-color: var(--bg-primary); color: var(--text-primary); border-color: var(--border)" required>
@@ -146,6 +147,19 @@ templ AdminLibrary(user User, libraries []LibraryData, users []User) {
             </div>
         </div>
 
+        <!-- Folder Browser Modal -->
+        <div id="folder-browser-modal" class="hidden fixed inset-0 z-50 flex items-center justify-center" style="background-color: rgba(0, 0, 0, 0.7);">
+            <div class="card rounded-lg p-6 w-full max-w-md mx-4" style="background-color: var(--bg-secondary); border-color: var(--border);">
+                <div class="flex justify-between items-center mb-4">
+                    <h2 class="text-xl font-bold" style="color: var(--text-primary)">Browse Folders</h2>
+                    <button type="button" data-action="browse-cancel" class="p-2 hover:opacity-80 rounded" style="color: var(--text-primary)">✕</button>
+                </div>
+                <div id="folder-browser-content">
+                    <!-- Directory listings will be rendered here -->
+                </div>
+            </div>
+        </div>
+
         <script src="/static/htmx.min.js"></script>
         <script src="/static/toast.js"></script>
         <script src="/static/api.js"></script>