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.

src/greeter.rs

@@ -6,6 +6,7 @@ use gdk_pixbuf::Pixbuf;
 use glib::clone;
 use gtk4::prelude::*;
 use gtk4::{self as gtk, gio};
+use image::imageops;
 use std::cell::RefCell;
 use std::collections::HashMap;
 use std::os::unix::net::UnixStream;
@@ -96,9 +97,10 @@ fn is_valid_username(name: &str) -> bool {
 }
 
 /// Load the background image as a shared texture (decode once, reuse everywhere).
-pub fn load_background_texture(bg_path: &Path) -> Option<gdk::Texture> {
+/// When `blur_radius` is `Some(sigma)` with sigma > 0, a Gaussian blur is applied.
+pub fn load_background_texture(bg_path: &Path, blur_radius: Option<f32>) -> Option<gdk::Texture> {
     let path_str = bg_path.to_str()?;
-    if bg_path.starts_with("/dev/moonarch/moongreet") {
+    let texture = if bg_path.starts_with("/dev/moonarch/moongreet") {
         match gio::resources_lookup_data(path_str, gio::ResourceLookupFlags::NONE) {
             Ok(bytes) => match gdk::Texture::from_bytes(&bytes) {
                 Ok(texture) => Some(texture),
@@ -132,9 +134,37 @@ pub fn load_background_texture(bg_path: &Path) -> Option<gdk::Texture> {
                 None
             }
         }
+    }?;
+
+    match blur_radius {
+        Some(sigma) if sigma > 0.0 => Some(apply_blur(&texture, sigma)),
+        _ => Some(texture),
     }
 }
 
+/// Apply Gaussian blur to a texture and return a blurred texture.
+fn apply_blur(texture: &gdk::Texture, sigma: f32) -> gdk::Texture {
+    let width = texture.width() as u32;
+    let height = texture.height() as u32;
+    let stride = width as usize * 4;
+    let mut pixel_data = vec![0u8; stride * height as usize];
+    texture.download(&mut pixel_data, stride);
+
+    let img = image::RgbaImage::from_raw(width, height, pixel_data)
+        .expect("pixel buffer size matches texture dimensions");
+    let blurred = imageops::blur(&image::DynamicImage::ImageRgba8(img), sigma);
+
+    let bytes = glib::Bytes::from(blurred.as_raw());
+    let mem_texture = gdk::MemoryTexture::new(
+        width as i32,
+        height as i32,
+        gdk::MemoryFormat::B8g8r8a8Premultiplied,
+        &bytes,
+        stride,
+    );
+    mem_texture.upcast()
+}
+
 /// Create a wallpaper-only window for secondary monitors.
 pub fn create_wallpaper_window(
     texture: &gdk::Texture,
@@ -1628,7 +1658,7 @@ mod tests {
 
     #[test]
     fn load_background_texture_missing_file_returns_none() {
-        let result = load_background_texture(Path::new("/nonexistent/wallpaper.jpg"));
+        let result = load_background_texture(Path::new("/nonexistent/wallpaper.jpg"), None);
         assert!(result.is_none());
     }
 
@@ -1639,7 +1669,7 @@ mod tests {
         // Create a sparse file that exceeds MAX_WALLPAPER_FILE_SIZE
         let f = std::fs::File::create(&path).unwrap();
         f.set_len(MAX_WALLPAPER_FILE_SIZE + 1).unwrap();
-        let result = load_background_texture(&path);
+        let result = load_background_texture(&path, None);
         assert!(result.is_none());
     }
 
@@ -1650,7 +1680,7 @@ mod tests {
         // 0xFF is not valid UTF-8
         let non_utf8 = OsStr::from_bytes(&[0xff, 0xfe, 0xfd]);
         let path = Path::new(non_utf8);
-        let result = load_background_texture(path);
+        let result = load_background_texture(path, None);
         assert!(result.is_none());
     }
 }