feat(ui): hide management controls for system collections

System collections (Not Started, Continue Reading, etc.) compute
their contents dynamically from reading_progress, so manual
add/remove has no effect. Hide the search bar, Remove Selected
button, Add Books button, per-card checkboxes, and Remove buttons
when the collection is system, making the page visually read-only.

- Add IsSystem bool to CollectionData, populated from the database
  is_system_collection flag.
- Add data-is-system to #collection-data so the JS renderer can
  also conditionally omit controls on library switch.
- Wrap toolbar controls, book picker modal, card checkboxes, and
  remove buttons in if !collection.IsSystem in the template.

internal/router/frontend.go

@@ -631,6 +631,7 @@ func registerFrontendRoutes(cfg *Config) {
 			Description: collection.Description.String,
 			Color:       collection.Color.String,
 			Icon:        collection.Icon.String,
+			IsSystem:    collection.IsSystemCollection.Bool,
 		}
 
 		var buf bytes.Buffer