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

internal/router/frontend.go

@@ -810,8 +810,9 @@ func registerFrontendRoutes(cfg *Config) {
 		if err != nil {
 			return renderErrorPage(c, "Error loading user", "user_load_error")
 		}
+		stats := getAdminStats(c.Request().Context(), cfg)
 		var buf bytes.Buffer
-		err = templates.Admin(user).Render(c.Request().Context(), &buf)
+		err = templates.Admin(user, stats).Render(c.Request().Context(), &buf)
 		if err != nil {
 			return err
 		}
@@ -823,8 +824,9 @@ func registerFrontendRoutes(cfg *Config) {
 		if err != nil {
 			return renderErrorPage(c, "Error loading user", "user_load_error")
 		}
+		stats := getAdminStats(c.Request().Context(), cfg)
 		var buf bytes.Buffer
-		err = templates.Admin(user).Render(c.Request().Context(), &buf)
+		err = templates.Admin(user, stats).Render(c.Request().Context(), &buf)
 		if err != nil {
 			return err
 		}
@@ -848,11 +850,14 @@ func registerFrontendRoutes(cfg *Config) {
 		libData := make([]templates.LibraryData, len(libraries))
 		for i, lib := range libraries {
 			libUUID, _ := uuid.FromBytes(lib.ID.Bytes[0:16])
+			folders, _ := cfg.LibraryService.GetLibraryFolders(c.Request().Context(), lib.ID)
 			libData[i] = templates.LibraryData{
 				ID:          libUUID.String(),
 				Name:        lib.Name,
 				Description: getText(lib.Description),
 				TypeName:    lib.TypeName,
+				TypeValue:   lib.TypeName,
+				FolderCount: len(folders),
 			}
 		}
 
@@ -1046,6 +1051,9 @@ func registerFrontendRoutes(cfg *Config) {
 		return c.HTML(http.StatusOK, buf.String())
 	}))
 
+	// Admin library HTMX endpoints
+	registerAdminLibraryRoutes(cfg, frontendProtected)
+
 	// ============================================================================
 	// LEGACY API ROUTES (for backward compatibility)
 	// ============================================================================