feat(windows): mpv plays audio on Windows, with libmpv shipped in the installer

libmpv on Windows (DR-237):
- The builder image carries zhongfly's LGPL libmpv-2.dll, pinned by
  asset name and sha256, plus an MSVC mpv.lib generated from the DLL's
  own mpv_* exports (the archive ships only a MinGW .dll.a). LGPL, not
  the GPL builds: no x264/x265, mpv -Dgpl=false; FFmpeg is version3, so
  LGPL-3.0. THIRD_PARTY_NOTICES.md records it.
- build-windows-cross.sh stages both files into src-tauri/windows-libs/;
  build.rs links mpv.lib from there and tauri.windows.conf.json bundles
  the DLL beside jellytau.exe from there, with the licence texts under
  licenses/. One directory, so the DLL shipped is the one linked.
- Workflows move to builder image 2026.09.1.

Windows audio:
- MpvBackend replaces WebviewAudioBackend on Windows (ao=wasapi), so
  volume, EQ, normalization and gapless work there as on Linux. Local
  files are passed to mpv as native paths, not file:// URLs.

Fixes found on the way:
- player_play_item decided "does the backend render video" with
  cfg!(not(linux)), true on Windows, while get_player_status sent
  Windows video to the <video> element. With mpv as the backend that
  would decode every film's soundtrack twice. All three callers now
  ask video_renders_natively() (UT-273).
- confine_queued_path rebuilt paths with PathBuf::push, so on Windows
  a queued `downloads/x` was stored as `downloads\x`, no longer the
  spelling the app built. It now keeps the caller's separator (UT-205,
  which only ever ran on Linux, failed under Windows).

Verified: jellytau.exe imports libmpv-2.dll; the full unit suite
cross-compiled for Windows passes under wine against the shipped DLL
(943/943, including the mpv injection and TLS tests); the NSIS
installer contains the DLL and licence texts. Not yet run on real
Windows hardware.

src-tauri/src/player/mpv_backend.rs

@@ -9,7 +9,6 @@ use crate::utils::conversions::{seconds_to_ticks, volume_to_percent};
 use crate::utils::lock::MutexSafe;
 use libmpv::Mpv;
 use log::{debug, error, info, warn};
-use std::process::Command;
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
@@ -54,8 +53,21 @@ struct InternalState {
     volume: f32,
 }
 
-/// Detect which audio system is available on the system
+/// The audio output mpv should use on Windows: WASAPI, the only one it ships
+/// there. Nothing to probe — and spawning `pactl` from a GUI app on Windows
+/// would at best fail and at worst flash a console window.
+///
+/// TRACES: UR-004 | DR-237
+#[cfg(target_os = "windows")]
 fn detect_audio_system() -> String {
+    "wasapi".to_string()
+}
+
+/// Detect which audio system is available on the system
+#[cfg(not(target_os = "windows"))]
+fn detect_audio_system() -> String {
+    use std::process::Command;
+
     info!("[MpvBackend] Detecting audio system...");
 
     // Try PulseAudio/PipeWire first (most common on modern Linux)
@@ -95,6 +107,13 @@ fn detect_audio_system() -> String {
 fn get_stream_url(media: &MediaItem) -> String {
     match &media.source {
         MediaSource::Remote { stream_url, .. } => stream_url.clone(),
+        // A Windows path is not a URL (`file://C:\...` is malformed); mpv
+        // takes the native path as it is. Safe to pass verbatim because it goes
+        // to mpv as one argv element (DR-298), not through a command string.
+        // TRACES: UR-004, UR-071 | DR-237
+        MediaSource::Local { file_path, .. } if cfg!(target_os = "windows") => {
+            file_path.to_string_lossy().into_owned()
+        }
         MediaSource::Local { file_path, .. } => {
             format!("file://{}", file_path.to_string_lossy())
         }
@@ -121,6 +140,8 @@ static MPV_HANDLE: std::sync::OnceLock<usize> = std::sync::OnceLock::new();
 /// can fail, and the app falls back to a no-op backend rather than dying).
 ///
 /// TRACES: UR-080 | DR-231
+// Only the Linux video surface reads it until Windows gets one (DR-237).
+#[cfg_attr(not(target_os = "linux"), allow(dead_code))]
 pub fn registered_handle() -> *mut libmpv_sys::mpv_handle {
     MPV_HANDLE
         .get()