viewer: default mpv to --hwdec=auto

Software H.264 decode at 1080p struggled on a low-power viewer (Intel
UHD 620 laptop on battery / power-save governor), surfacing as choppy
motion and A-V drift even though the host encode side was clean. mpv's
auto hwdec picks vaapi/nvdec/etc. when available and silently falls
back to software when not, so the default is strictly safer.

Applied to both the headless banner recipe and the interactive
player-picker spawn args.

src/interactive.rs

@@ -61,6 +61,7 @@ impl Player {
                 &[
                     "--profile=low-latency",
                     "--untimed",
+                    "--hwdec=auto",
                     "--audio-buffer=0.2",
                     "--demuxer-max-bytes=2M",
                     "--demuxer-readahead-secs=0.5",

src/viewer/mod.rs

@@ -56,7 +56,7 @@ fn print_viewer_banner(url: &str) {
     eprintln!("│  Connected to host. Open the stream in your player:");
     eprintln!("│");
     eprintln!(
-        "│    mpv --profile=low-latency --untimed --audio-buffer=0.2 --demuxer-max-bytes=2M --demuxer-readahead-secs=0.5 {url}"
+        "│    mpv --profile=low-latency --untimed --hwdec=auto --audio-buffer=0.2 --demuxer-max-bytes=2M --demuxer-readahead-secs=0.5 {url}"
     );
     eprintln!("│    vlc --network-caching=200 --live-caching=200 {url}");
     eprintln!("│");