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>

crates/yaak/Cargo.toml

@@ -9,6 +9,7 @@ async-trait = "0.1"
 base64 = "0.22.1" # For carrying body chunks over a text-only plugin transport
 log = { workspace = true }
 md5 = "0.8.0"
+sha2 = { workspace = true }
 chrono = { workspace = true }
 serde_json = { workspace = true }
 thiserror = { workspace = true }