feat(core): support ai chat delete action (#13655)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- New Features
- Delete icon added to AI session history with tooltip and confirmation
prompt; deleting current session opens a new session.
- Session deletion wired end-to-end (toolbar → provider → backend) and
shows notifications.

- Improvements
- Cleanup now supports deleting sessions with or without a document ID
(document-specific or workspace-wide).
- UI tweaks for cleaner session item layout and safer click handling
(delete won’t trigger item click).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/schema.gql

@@ -542,7 +542,7 @@ type DeleteAccount {
 }
 
 input DeleteSessionInput {
-  docId: String!
+  docId: String
   sessionIds: [String!]!
   workspaceId: String!
 }