feat(import): remember the user's import selection

Yaak now remembers, per item of a linked import source, whether the user
wants it. A mapping row that still points at a model means wanted, so
re-imports merge into it; a row with no model behind it means not wanted,
so re-imports leave it alone. Wanted-ness is structural rather than a flag:
deleting the model locally, or turning the item down in the preview, is
what makes it not wanted.

- import_source_resources drops `snapshot` for a nullable `content_hash`
  and lets `model_id` be NULL. The migration keeps every beta row's
  key to model mapping and starts hashes empty; until a hash is recorded,
  a difference can't be attributed to either side, so the item is offered
  once as a conflict that keeps local changes.
- comparable() ignores sortPriority. Importers number it from source
  order, so inserting one operation used to plan a fake update for
  everything after it. The trade is that pure reorders don't propagate.
- Keys the user turned down come back as unchecked "not imported" rows
  instead of being re-offered as new resources or resurrected. Checking
  one imports it under the same source key and pulls in the folders it
  needs.
- An imported resource the source moves into a folder that isn't imported
  is offered as a deletion, with a tooltip pointing at the folder.
- Plan-time source resolution binds by source-key overlap instead of by
  path, so a renamed or moved file merges into what it created and heals
  the stored origin. Several sources sharing keys never guess-merge: the
  plan says so and imports everything as new.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/yaak-client/components/core/CheckboxTree.tsx

@@ -21,6 +21,8 @@ interface Props<T> {
   isCheckboxDisabled?: (node: CheckboxTreeNode<T>) => boolean;
   /** An irrelevant row is hidden unless one of its descendants is relevant */
   isRelevant: (node: CheckboxTreeNode<T>) => boolean;
+  /** A node that starts collapsed, so a large subtree doesn't crowd out the rest */
+  isCollapsedByDefault?: (node: CheckboxTreeNode<T>) => boolean;
   renderRow: (node: CheckboxTreeNode<T>) => ReactNode;
   onSelectRow?: (node: CheckboxTreeNode<T>) => void;
   canSelectRow?: (node: CheckboxTreeNode<T>) => boolean;
@@ -29,7 +31,9 @@ interface Props<T> {
 
 export function CheckboxTree<T>(props: Props<T>) {
   const { node, depth = 0 } = props;
-  const [collapsed, setCollapsed] = useState<boolean>(false);
+  const [collapsed, setCollapsed] = useState<boolean>(
+    () => props.isCollapsedByDefault?.(node) ?? false,
+  );
   if (!hasRelevantNode(node, props.isRelevant)) return null;
 
   const checked = props.checked(node);