Eng-council sweep: export hygiene, dead code, helper dedup, declare vega-expression

src/core/storage-estimate.ts

@@ -16,7 +16,7 @@
  */
 
 /** A category of stored data and its measured size in bytes. */
-export interface StorageSegment {
+interface StorageSegment {
   key: 'snippets' | 'datasets' | 'app';
   /** User-facing label, centralized here so the component stays presentational. */
   label: string;