refactor: rename listed → discoverable on note

Pure rename — no behavioral change. "discoverable" more clearly
communicates that the field controls whether a note can be found
by others in public listings.

src/data/note.ts

@@ -4,5 +4,5 @@ export type Note = {
   title: string;
   publishedAt: string;
   createdAt: string;
-  listed?: boolean;
+  discoverable?: boolean;
 };