screenshare: honor viewer settings for the VLC player too

The viewer playback settings (hardware decode + buffering) only shaped
mpv's argv; vlc_args() was fixed, so a VLC viewer silently ignored them.
The load-bearing case is hardware decode: mpv defaults to software decode
(the A-bug fix), but VLC hardware-decodes by default, so a VLC viewer with
the default hardware_decode=false still got GPU decode and could hit the
frame-1 freeze the default exists to avoid — the toggle did nothing.

vlc_args() now takes the settings and maps the knobs that translate
cleanly to VLC: hardware decode (--avcodec-hw=none/any) and buffering
posture (network/live caching ms). The genuinely mpv-specific knobs
(cache_mb byte-cache, extra_mpv_args) stay mpv-only; the Settings UI
hints are reworded to say which knobs are mpv-only vs universal. +2 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -3452,7 +3452,7 @@ fn share_player_hint(player: SharePlayer) -> &'static str {
 fn share_buffering_hint(buffering: ShareBuffering) -> &'static str {
     match buffering {
         ShareBuffering::LowLatency => "Small buffers for interactive screen sharing.",
-        ShareBuffering::Smooth => "Larger mpv cache/readahead for steadier playback.",
+        ShareBuffering::Smooth => "Larger cache/readahead for steadier playback.",
     }
 }
 
@@ -5146,13 +5146,13 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
                             Some(screen_share.cache_mb),
                             AppMessage::ScreenShareCacheMbSelected,
                         ).width(iced::Length::Fill),
-                        text("Used as mpv demuxer cache size.").size(11).color(color_subtext),
+                        text("mpv demuxer cache size (mpv only).").size(11).color(color_subtext),
                     ].spacing(4).width(iced::Length::Fill),
                     column![
                         checkbox(screen_share.hardware_decode)
                             .label("Hardware video decode")
                             .on_toggle(AppMessage::ToggleScreenShareHardwareDecode),
-                        text("Adds --hwdec=auto to mpv. Off avoids the known frame-freeze bug.").size(11).color(color_subtext),
+                        text("GPU decode (mpv --hwdec=auto / VLC hardware decode). Off avoids the known frame-freeze bug.").size(11).color(color_subtext),
                     ].spacing(8).width(iced::Length::Fill),
                 ].spacing(16).width(iced::Length::Fill),
             ].spacing(10).width(iced::Length::Fill),