feat: add optional background blur via image crate

Gaussian blur applied at texture load time when `background_blur` is
set in moonset.toml. Blur runs once, result is shared across monitors.

DECISIONS.md

@@ -2,6 +2,13 @@
 
 Architectural and design decisions for Moonset, in reverse chronological order.
 
+## 2026-03-28 – Optional background blur via `image` crate
+
+- **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.
+
 ## 2026-03-28 – Use absolute paths for system binaries
 
 - **Who**: Hekate, Dom