docs(collections): Add user documentation for library filtering

- Document collection viewing from collections page vs dashboard
- Explain library filtering behavior with query parameters
- Clarify backward compatible behavior (no filter = all books)

docs/user/collections.md

@@ -0,0 +1,26 @@
+# Collections
+
+Collections allow you to organize books across multiple libraries.
+
+## Viewing Collections
+
+### From Collections Page
+
+Navigate to `/collections` to see all your collections. Clicking on a collection shows ALL books in that collection across all libraries.
+
+### From Dashboard
+
+When viewing a specific library's dashboard, collections only show books from that library. Clicking "View All" preserves this context and shows only books from that library in the collection.
+
+### Library Filtering
+
+- **No filter** (`/collections/favorites`): Shows all books across all libraries
+- **With filter** (`/collections/favorites?library_id=xxx`): Shows only books from the specified library
+
+## Creating Collections
+
+[Instructions for creating collections]
+
+## Managing Collections
+
+[Instructions for editing/deleting collections]