refactor(ui): one rule for resettable, role key and display name; an honest reset label

The card menu's reset read 'Clear watch progress' but also un-hides and un-watches; it now says so.
The resettable rule, the demo-first role key and the display-name fallback each lived in several
components tied by comments; they are shared helpers now, and the menu reuses the card's own
strings instead of duplicate keys.

frontend/src/lib/api/feed.ts

@@ -28,6 +28,12 @@ export interface Video {
   watch_url: string;
 }
 
+/** Whether a video has anything its reset would clear: pristine = never opened, the default status
+ *  and no resume position. The card's reset button and the card menu's reset both ask this. */
+export function isResettable(v: Pick<Video, "status" | "position_seconds">): boolean {
+  return v.status !== "new" || v.position_seconds > 0;
+}
+
 export interface VideoDetail {
   id: string;
   description: string | null;