feat(FN-4238): export merger errors from engine and polish workflow results

Exports merger error types from the engine package for use in the dashboard and CLI. Adds tests for the engine barrel exports and fixes workflow results tab styling to align the edit toggle with button styles.

Fusion-Task-Id: FN-4238

packages/engine/src/index.ts

@@ -28,6 +28,9 @@ export {
   dropAutostashBySha,
   getAutostashDiff,
   notifyAutostashOrphans,
+  DiffVolumeRegressionError,
+  MergeAbortedError,
+  SquashAuditError,
   type MergerOptions,
   type AutostashOrphanRecord,
 } from "./merger.js";