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/account.ts

@@ -35,6 +35,17 @@ export interface Me {
   diagnostics: DiagnosticsState;
 }
 
+/** The `nav.role.*` key an account is shown under. Demo isn't a real role (it's a flag on a `user`
+ *  row), so it comes first. */
+export function roleKey(a: Pick<Me, "is_demo" | "role">): "admin" | "user" | "demo" {
+  return a.is_demo ? "demo" : a.role === "admin" ? "admin" : "user";
+}
+
+/** What an account is called on screen: its display name, else the local part of its email. */
+export function displayName(a: { display_name: string | null; email: string }): string {
+  return a.display_name ?? a.email.split("@")[0]!;
+}
+
 export interface Account {
   id: number;
   email: string;

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;