feat(admin): redesign library management with HTMX expandable rows + stats dashboard

Library management:
- Redesign admin_library page with expandable rows that load detail
  panels via HTMX (LibraryList, LibraryPanel, FolderBrowserContent partials)
- Add create/edit/delete library modals using data-* attributes
- Add folder browser modal with inline add/remove via HTMX
- Add user visibility checkboxes toggled via HTMX
- New endpoints in admin_library.go: create, update, delete, panel,
  folders add/remove, browse, visibility
- New template types: FolderData, DirEntry, UserVisibilityData,
  AdminStats; extend LibraryData with TypeValue and FolderCount

Admin dashboard:
- Rewrite admin.templ to show 4-stat grid (libraries, media, users, devices)
- Add getAdminStats helper querying library/user/media/device counts
- Pass AdminStats to template from both /admin and /admin/ handlers

templates/types.go

@@ -38,7 +38,32 @@ type LibraryData struct {
 	Name        string
 	Description string
 	TypeName    string
+	TypeValue   string
 	MediaCount  int64
+	FolderCount int
+}
+
+type FolderData struct {
+	FolderPath string
+}
+
+type DirEntry struct {
+	Name string
+	Path string
+}
+
+type UserVisibilityData struct {
+	UserID     string
+	Username   string
+	Email      string
+	IsVisible  bool
+}
+
+type AdminStats struct {
+	LibraryCount int
+	MediaCount   int
+	UserCount    int
+	DeviceCount  int
 }
 
 type SeriesCardData struct {