test(player): hold the session stop and the kept-point rule to the server

- The client's resume thresholds equal the server's (read from both sources).
- The session stop's URL matches the route the server serves it on.
- The player stops its server session by its item and its own id on an unload and on close,
  and keeps it when the page only goes into the back/forward cache.

frontend/src/lib/resume.ts

@@ -6,8 +6,8 @@
 // Where the point is kept is the caller's: the library's on the server (`library_states`), a shared
 // link's in this browser (the viewer has no account). The rule for which point is worth keeping is
 // the server's own (`routes/_common.kept_position`), repeated here for the link that has no server
-// side: too early to count as started, or close enough to the end to count as finished, is "from
-// the start".
+// side (`resume.contract.test.ts` holds the two numbers equal): too early to count as started, or
+// close enough to the end to count as finished, is "from the start".
 import { useEffect, useEffectEvent } from "react";
 
 import { LS, readJSON, writeJSON } from "./storage";