refactor(plex): the item progress body extends the shared checkpoint

ItemProgressIn repeated ProgressIn's config, fields and bounds; it now adds only `final`. The
threshold comment points where the constants live now, and plexProgressBeacon's comment is back
on it.

frontend/src/lib/api/plex.ts

@@ -720,10 +720,6 @@ export const plexApi = {
       method: "POST",
       body: JSON.stringify({ position_seconds, duration_seconds, final }),
     }),
-  // Fire-and-forget progress save that SURVIVES page unload (F5/close/navigate): a normal fetch is
-  // cancelled on unload and React effect cleanup doesn't run on a full reload, so the last position
-  // (esp. right after a seek) would be lost. `keepalive` lets the POST complete during unload; we
-  // replicate req()'s credentials + account header (no retry — there's no page left to retry on).
   // The player no longer wants this HLS session: its ffmpeg stops now instead of at the idle reaper.
   // A keepalive POST, so it also goes out from a page being unloaded. The server only stops it while
   // it is still the viewer's registered session for the item.
@@ -732,6 +728,10 @@ export const plexApi = {
       `/api/plex/stream/${encodeURIComponent(id)}/session/${encodeURIComponent(sessionId)}/stop`,
       {},
     ),
+  // Fire-and-forget progress save that SURVIVES page unload (F5/close/navigate): a normal fetch is
+  // cancelled on unload and React effect cleanup doesn't run on a full reload, so the last position
+  // (esp. right after a seek) would be lost. `keepalive` lets the POST complete during unload; we
+  // replicate req()'s credentials + account header (no retry — there's no page left to retry on).
   plexProgressBeacon: (
     id: string,
     position_seconds: number,