Files: a sweep of the edges

Found by walking every handler and the UI with odd inputs, then a battery of requests
against the live hub. Fixed:
- A save over the 8 MiB cap was cut short in place: the file was truncated, then the
  first 8 MiB written, and the caller told 204. Now the body is read whole first and a
  body over the cap is a 413 with the file untouched.
- A link to a file showed the link's own time; now the file's, so the row reads right
  and the preview key follows the file.
- Reading a link to nothing answered 200 with an empty body (the open failed after the
  headers in root mode); now a 404 that names the target.
- A play request with an unknown codec or size was answered by inspecting the file
  first, so an audio-only file said "no video stream" instead of what was wrong; the
  choice is checked before any work, and is a 400.
- A stream that ended in failure was handed back to the next identical request for ten
  minutes; a failed one is now dropped and a fresh one made.
- A player that left while its stream was still finding a first segment made the loop
  try the next recipe; it stops. A stream closed mid-start is not started again.
- Ending a root-mode ffmpeg could wait forever: the daemon cannot signal root's
  processes, so the wait is bounded and the helper's own grace does the rest.
- A page going away (a phone closing the tab) releases its stream by a beacon, rather
  than ten minutes later by the sweeper.
- The path box resolves like a shell — relative to the folder shown, . and .. applied,
  trailing slashes dropped — in the tab and in the Move picker, with a test. A typed path
  that is a file opens it; a remembered folder that is gone falls back to / once.
- A dropped folder arrived as a nameless empty file; it is refused with a word. The
  upload toast counts what was actually uploaded.
- The viewer names a device, pipe or socket, and a dangling link, instead of calling
  them binary files.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/api/play.go

@@ -79,6 +79,10 @@ func (s *Server) playStart(w http.ResponseWriter, r *http.Request) {
 		jsonErr(w, 415, "not a video")
 		return
 	}
+	if err := media.ValidChoice(body.Codec, body.Quality); err != nil {
+		jsonErr(w, 400, err.Error())
+		return
+	}
 	if _, err := fsys.Stat(r.Context(), p); err != nil {
 		fsErr(w, err)
 		return
@@ -115,6 +119,8 @@ func playErr(w http.ResponseWriter, err error) {
 		jsonErr(w, 422, de.Msg)
 	case strings.Contains(err.Error(), "already being re-encoded"):
 		jsonErr(w, 429, err.Error())
+	case strings.HasPrefix(err.Error(), "unknown ") || strings.HasPrefix(err.Error(), "nothing on this machine encodes"):
+		jsonErr(w, 400, err.Error())
 	case errors.Is(err, context.DeadlineExceeded):
 		jsonErr(w, 504, "the file took too long to read")
 	default:
@@ -132,7 +138,8 @@ func (s *Server) playStatus(w http.ResponseWriter, r *http.Request) {
 	jsonOK(w, 200, st.Status())
 }
 
-// playStop: DELETE /api/files/play/{id}
+// playStop: DELETE /api/files/play/{id}, or POST …/close — the form a page can send on its
+// way out (navigator.sendBeacon), so a phone closing the tab releases the machine at once.
 func (s *Server) playStop(w http.ResponseWriter, r *http.Request) {
 	s.plays.Close(r.PathValue("id"))
 	w.WriteHeader(204)