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-models/src/models.rs

@@ -3118,8 +3118,12 @@ pub struct ImportSourceResource {
     pub import_source_id: String,
     pub source_key: String,
     pub model_type: String,
-    pub model_id: String,
-    pub snapshot: String,
+    /// `None` once the user has decided not to import this key
+    #[ts(optional)]
+    pub model_id: Option<String>,
+    /// Hash of the resource as last applied or decided from the source, if one was recorded
+    #[ts(optional)]
+    pub content_hash: Option<String>,
 }
 
 impl<'s> TryFrom<&Row<'s>> for ImportSourceResource {
@@ -3134,7 +3138,7 @@ impl<'s> TryFrom<&Row<'s>> for ImportSourceResource {
             source_key: r.get("source_key")?,
             model_type: r.get("model_type")?,
             model_id: r.get("model_id")?,
-            snapshot: r.get("snapshot")?,
+            content_hash: r.get("content_hash")?,
         })
     }
 }