fix(grid): persist WHERE filter input across tab switches (closes #210)

whereFilterInput was a component-local ref that got lost when switching
tabs because DataGrid is destroyed and recreated via :key binding.

Store whereInput in the QueryTab state and pass it as initialWhereInput
prop so it survives tab switches.

src/types/database.ts

@@ -251,6 +251,7 @@ export interface QueryTab {
     selectStar: boolean;
     columns: string[];
   };
+  whereInput?: string;
 }
 
 export interface SavedSqlFolder {