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.

templates/types.go

@@ -30,6 +30,7 @@ type CollectionData struct {
 	Description string
 	Color       string
 	Icon        string
+	IsSystem    bool
 }
 
 type LibraryData struct {