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.

scripts/build-windows-cross.sh

@@ -7,14 +7,16 @@
 # officially supports for Windows (the mingw/GNU target is not), and unlike GNU
 # it can bundle the NSIS installer from a Linux host.
 #
-# Playback on Windows: video renders via WebView2 and audio via the webview
-# <audio> backend (WebviewAudioBackend) — see docs/build/build-windows.md.
+# Playback on Windows: video renders via WebView2; audio via mpv, whose
+# libmpv-2.dll ships beside jellytau.exe — see docs/build/build-windows.md.
 #
 # Requirements (present in the Docker windows-cross target / unified builder):
 #   - rustup target x86_64-pc-windows-msvc
 #   - cargo-xwin           (cargo install --locked cargo-xwin)
 #   - lld, llvm            (linker + llvm-lib used by cargo-xwin)
 #   - nsis (makensis)      (installer generator)
+#   - libmpv for Windows   ($LIBMPV_WIN_DIR: libmpv-2.dll + mpv.lib, pinned and
+#                           sha256-checked in Dockerfile.builder)
 #
 # Usage:
 #   scripts/build-windows-cross.sh                    # exe + NSIS installer
@@ -29,10 +31,26 @@ WIN_BUNDLES="${WIN_BUNDLES:-nsis}"
 
 echo "🪟 Cross-compiling JellyTau for Windows ($TARGET, via cargo-xwin)"
 echo "================================================================"
-echo "Video plays via WebView2; audio via the webview <audio> backend."
+echo "Video plays via WebView2; audio via mpv (libmpv-2.dll)."
 echo "Bundles: $WIN_BUNDLES"
 echo ""
 
+# Stage libmpv where build.rs links it from and tauri.windows.conf.json bundles
+# it from — one directory, so the DLL shipped is the one linked against.
+# Copied fresh every build: a stale DLL left from an older image would ship
+# silently. TRACES: UR-004 | DR-237
+LIBMPV_WIN_DIR="${LIBMPV_WIN_DIR:-/opt/libmpv-win64}"
+for f in libmpv-2.dll mpv.lib; do
+  if [[ ! -f "$LIBMPV_WIN_DIR/$f" ]]; then
+    echo "❌ $LIBMPV_WIN_DIR/$f not found. Build inside the jellytau-builder image,"
+    echo "   or set LIBMPV_WIN_DIR to a directory holding libmpv-2.dll and mpv.lib."
+    exit 1
+  fi
+done
+rm -rf src-tauri/windows-libs
+mkdir -p src-tauri/windows-libs
+cp -v "$LIBMPV_WIN_DIR/libmpv-2.dll" "$LIBMPV_WIN_DIR/mpv.lib" src-tauri/windows-libs/
+
 bun install --frozen-lockfile 2>/dev/null || bun install
 bun run build