Add optional Parent Issue column to inbox show/hide columns

Adds a "parent" column option to the inbox column toggle dropdown.
When enabled, sub-issues display the parent's identifier (e.g. PAP-123)
with the parent title as a tooltip. Uses the existing issueById lookup
map to resolve parent info without additional API calls.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/pages/Inbox.test.tsx

@@ -205,6 +205,8 @@ describe("InboxIssueTrailingColumns", () => {
           workspaceName={null}
           assigneeName={null}
           currentUserId={null}
+          parentIdentifier={null}
+          parentTitle={null}
         />,
       );
     });
@@ -229,6 +231,8 @@ describe("InboxIssueTrailingColumns", () => {
           workspaceName={null}
           assigneeName={null}
           currentUserId={null}
+          parentIdentifier={null}
+          parentTitle={null}
         />,
       );
     });