fix: restore booklet viewer with mupdf 0.6 (#170)

mupdf 0.5 had struct size mismatches on Arch Linux (mupdf-sys bindings
incompatible with system MuPDF headers). Upgrading to mupdf 0.6 fixes
the bindings.

- Removed the booklet feature gate — mupdf is now a direct dependency
  again, matching the v1.1.19 configuration
- Removed stub implementation (always compile real MuPDF backend)
- sys-lib-libjpeg remains excluded (cross-distro libjpeg incompatibility)

cc

src-tauri/Cargo.toml

@@ -13,10 +13,6 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 name = "qbz"
 path = "src/main.rs"
 
-[features]
-default = []
-booklet = ["mupdf"]
-
 [build-dependencies]
 tauri-build = { version = "2", features = [] }
 
@@ -107,7 +103,7 @@ image = { version = "0.25", default-features = false, features = ["png", "jpeg",
 # 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 = "0.5", default-features = false, optional = true, features = [
+mupdf = { version = "0.6", default-features = false, features = [
   "system-fonts",
   "sys-lib-zlib",
   "sys-lib-harfbuzz",