feat(feed): resume progress bar, play/continue/restart, in-progress filter

Video cards show a resume progress bar for started-but-unfinished videos and a
hover overlay: Play on every card, Continue + Restart on in-progress ones. The
in-app player now resumes from (and checkpoints to) the server position instead
of localStorage, accepts an explicit startAt (Restart -> 0), and refreshes the
feed on close so the card bar reflects the session. Sidebar gains an
'In progress' show filter.

frontend/src/components/Sidebar.tsx

@@ -47,6 +47,7 @@ const SORTS = [
 
 const SHOWS = [
   { id: "unwatched", label: "Unwatched" },
+  { id: "in_progress", label: "In progress" },
   { id: "all", label: "All" },
   { id: "watched", label: "Watched" },
   { id: "saved", label: "Saved" },