task 9.3: create src/types/charts.ts with Chart interface

Adds a shared Chart interface matching the DB schema (id, name, created_at)
and the existing usages in page.tsx and ChartSelector.tsx.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/types/charts.ts

@@ -0,0 +1,12 @@
+/**
+ * Chart types for chart metadata.
+ *
+ * created_at is stored as a PostgreSQL timestamp (returned as Date by Drizzle ORM)
+ * but is often treated as a Unix epoch number in the frontend after JSON serialisation.
+ */
+
+export interface Chart {
+  id: number;
+  name: string;
+  created_at: Date | number;
+}