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/server.go

@@ -317,6 +317,7 @@ func (s *Server) routes() *http.ServeMux {
 	m.HandleFunc("POST /api/files/play", s.auth(s.playStart))
 	m.HandleFunc("GET /api/files/play/{id}", s.auth(s.playStatus))
 	m.HandleFunc("DELETE /api/files/play/{id}", s.auth(s.playStop))
+	m.HandleFunc("POST /api/files/play/{id}/close", s.auth(s.playStop))
 	m.HandleFunc("GET /api/files/play/{id}/{file}", s.auth(s.playFile))
 	m.HandleFunc("GET /api/hardware", s.auth(s.hardwareReport))
 	m.HandleFunc("GET /api/network", s.auth(s.networkOverview))
@@ -1931,6 +1932,10 @@ func (s *Server) filesRead(w http.ResponseWriter, r *http.Request) {
 		jsonErr(w, 400, "not a regular file (device, pipe or socket)")
 		return
 	}
+	if in.Broken {
+		jsonErr(w, 404, "a link to nothing: "+in.Target)
+		return
+	}
 	mx, _ := strconv.ParseInt(r.URL.Query().Get("max"), 10, 64)
 	if mx > 0 && in.Size > mx {
 		jsonErr(w, 413, "file too large")
@@ -1989,7 +1994,14 @@ func (s *Server) filesWrite(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	if err := fsys.Write(r.Context(), p, io.LimitReader(r.Body, 8<<20)); err != nil {
+	// Whole in memory first: the write truncates the file, so a body over the cap must be
+	// refused before that, not cut short after it.
+	body, err := io.ReadAll(http.MaxBytesReader(w, r.Body, 8<<20))
+	if err != nil {
+		jsonErr(w, 413, "too large to save here (8 MiB at most)")
+		return
+	}
+	if err := fsys.Write(r.Context(), p, bytes.NewReader(body)); err != nil {
 		fsErr(w, err)
 		return
 	}