feat(collections): browse a collection from the sidebar

The sidebar gains the concept's COLLECTIONS group (D136): one row per
collection with its folder icon, name and listed-member count, without the
PROPOSED badge; the label shows once a collection exists. A row opens
#/collection/<id>, the Images page filtered to that collection (D137): the
collection's name as h1, Collection as breadcrumb and title, Edit to rename
or delete it through the saved views' Edit dialog with collection nouns, and
a note counting members the sources do not list now. An unknown or deleted
collection shows a page that says so with a link to Images. A saved view on
a deleted collection lists nothing and says so (MC6, D140). The list is read
again after a restore and the counts 300 ms after a change.

Part of #331

tests/browser/workspace_visual_fixture.py

@@ -112,6 +112,8 @@ def seed_workspace_visual(api: PromptManagerAPI, root: Path) -> None:
     """Replace only the owned browser harness records and media roots."""
     with api.db.model.get_connection() as conn:
         for table in (
+            "image_collection_items",
+            "image_collections",
             "saved_views",
             "image_favorites",
             "media_review_decisions",