fix(macos): enable x86_64 cross-compilation

- Upgrade rust_cast 0.18.1 → 0.21 (switches from openssl to rustls,
  removing the openssl-sys dependency that blocked x86_64 cross-compile)
- Split mupdf sys-lib features to Linux-only (on macOS there are no GTK
  symbol conflicts, and system libs don't support x86_64 cross-compile
  from Apple Silicon)
- Update release-macos workflow to build both aarch64 and x86_64 DMGs
  using a matrix strategy

src-tauri/Cargo.toml

@@ -99,16 +99,13 @@ souvlaki = "0.8"
 image = { version = "0.25", default-features = false, features = ["png", "jpeg", "webp"] }
 
 # PDF rendering with MuPDF engine (high-quality rasterization for booklet viewer)
-# NOTE: Use sys-lib-* for zlib/harfbuzz/freetype to avoid symbol conflicts with
-# GTK/GdkPixbuf. But NOT libjpeg — Ubuntu ships libjpeg8 (libjpeg.so.8) while
-# Debian/Fedora ship libjpeg62-turbo (libjpeg.so.62). Let mupdf vendor its own
-# libjpeg to avoid cross-distro breakage (GitHub #161, #162).
-mupdf = { version = "0.6", default-features = false, features = [
-  "system-fonts",
-  "sys-lib-zlib",
-  "sys-lib-harfbuzz",
-  "sys-lib-freetype",
-] }
+# NOTE: On Linux, use sys-lib-* for zlib/harfbuzz/freetype to avoid symbol
+# conflicts with GTK/GdkPixbuf. But NOT libjpeg — Ubuntu ships libjpeg8 while
+# Debian/Fedora ship libjpeg62-turbo. Let mupdf vendor its own libjpeg to avoid
+# cross-distro breakage (GitHub #161, #162).
+# On macOS, let mupdf vendor everything (no GTK conflicts, and system libs
+# don't support x86_64 cross-compilation from Apple Silicon).
+mupdf = { version = "0.6", default-features = false, features = ["system-fonts"] }
 
 # URL opening
 open = "5"
@@ -146,6 +143,7 @@ futures-util = "0.3"
 [target.'cfg(target_os = "linux")'.dependencies]
 tauri-plugin-dialog = { version = "2.6.0" }
 ashpd = { version = "0.13", default-features = false, features = ["tokio", "notification", "inhibit"] }
+mupdf = { version = "0.6", features = ["sys-lib-zlib", "sys-lib-harfbuzz", "sys-lib-freetype"] }
 
 # Direct ALSA access for bit-perfect playback (bypasses CPAL limitations)
 alsa = "0.9"