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.

DECISIONS.md

@@ -9,12 +9,12 @@ Architectural and design decisions for Moonset, in reverse chronological order.
 - **Tradeoffs**: Adds a field to `ActionDef` that most actions set to `false`. Acceptable because it makes the contract explicit and testable.
 - **How**: `ActionDef.quit_after: bool` — `true` for lock and logout, `false` for hibernate/reboot/shutdown.
 
-## 2026-03-28 – Optional background blur via `image` crate
+## 2026-03-28 – GPU blur via GskBlurNode replaces CPU blur
 
-- **Who**: Hekate, Dom
-- **Why**: Blurred wallpaper as background is a common UX pattern for overlay menus
-- **Tradeoffs**: Adds `image` crate dependency (~15 transitive crates); CPU-side Gaussian blur at load time adds startup latency proportional to image size and sigma. Acceptable because blur runs once and the texture is shared across monitors.
-- **How**: `load_background_texture(bg_path, blur_radius)` loads texture, optionally applies `imageops::blur()`, returns `gdk::Texture`. Config option `background_blur: Option<f32>` in TOML.
+- **Who**: Ragnar, Dom
+- **Why**: CPU-side Gaussian blur (`image` crate) blocked startup and added caching complexity. moonlock already migrated to GPU blur.
+- **Tradeoffs**: GPU blur quality is slightly different (box-blur approximation vs true Gaussian), acceptable for wallpaper backgrounds. Removes `image` crate dependency entirely. No disk cache needed.
+- **How**: `Snapshot::push_blur()` + `GskRenderer::render_texture()` on `connect_realize`. Blur happens once on the GPU when the widget gets its renderer. Symmetric with moonlock and moongreet.
 
 ## 2026-03-28 – Use absolute paths for system binaries