perf: replace CPU blur with GPU blur via GskBlurNode (v0.5.0)

Replace image crate + disk cache blur with GPU-side GskBlurNode,
symmetric with moonlock and moongreet. Removes ~15 transitive
dependencies and ~160 lines of caching code. Blur now happens once
on the GPU at widget realization — zero startup latency, no cache
management needed.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moonset"
-version = "0.4.1"
+version = "0.5.0"
 edition = "2024"
 description = "Wayland session power menu with GTK4 and Layer Shell"
 license = "MIT"
@@ -15,7 +15,7 @@ toml = "0.8"
 dirs = "6"
 serde = { version = "1", features = ["derive"] }
 nix = { version = "0.29", features = ["user"] }
-image = { version = "0.25", default-features = false, features = ["jpeg", "png"] }
+graphene-rs = { version = "0.22", package = "graphene-rs" }
 log = "0.4"
 env_logger = "0.11"