fix: omit height from NV12 capsfilter so GStreamer preserves source DAR

A height range like (int)[2,2160] includes the source height (1080),
so GStreamer's caps fixation picked identity for height (no scale) and
only scaled width 1920->720, giving a distorted 720x1080 frame.

Fix: omit height entirely from the capsfilter caps string.
GStreamer then derives the output height from the source's display
aspect ratio for the given width target.  NV12's even-dimension
requirement is satisfied automatically by caps fixation rounding.

src/r36s_dlna_browser/player/gstreamer_backend.py

@@ -461,13 +461,18 @@ class GStreamerBackend(PlayerBackend):
 
         # nearest-neighbour: accesses only the source pixels needed for each
         # output sample (strided reads), skipping ~56% of source rows entirely.
-        # Height is unconstrained (range 2–2160) so GStreamer picks whatever
+        # Height is intentionally OMITTED from the caps so GStreamer computes it
-        # height preserves the source aspect ratio for the chosen width.
+        # from the source's display aspect ratio (DAR).  Specifying a height
+        # range like (int)[2,2160] is wrong — GStreamer's caps fixation picks the
+        # nearest compatible value (the source height itself), bypassing scaling.
+        # Without any height cap, videoscale scales width to scale_w and derives
+        # the height that preserves the DAR; NV12's even-dimension requirement is
+        # satisfied automatically by GStreamer's caps fixation rounding.
         scale.set_property("method", 0)
         capsfilter.set_property(
             "caps",
             self._gst.Caps.from_string(
-                f"video/x-raw,format=NV12,width={scale_w},height=(int)[2,2160]"
+                f"video/x-raw,format=NV12,width={scale_w}"
             ),
         )