feat: add optional background blur via image crate

Gaussian blur applied at texture load time when `background-blur` is
set in the [appearance] section of moongreet.toml. Blur runs once,
result is shared across monitors.

DECISIONS.md

@@ -1,5 +1,12 @@
 # Decisions
 
+## 2026-03-28 – Optional background blur via `image` crate
+
+- **Who**: Selene, Dom
+- **Why**: Blurred wallpaper as greeter background is a common UX pattern for login screens
+- **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 blurred `gdk::Texture`. Config option `background-blur: Option<f32>` in `[appearance]` TOML section.
+
 ## 2026-03-28 – Audit fixes for shared wallpaper texture (v0.4.1)
 - **Who**: Selene, Dominik
 - **Why**: Quality, performance, and security audits flagged issues in `load_background_texture()`, debug logging, and greetd error handling