feat: add optional background blur, align to shared texture pattern

Gaussian blur applied at texture load time when `background_blur` is
set in moonlock.toml. Refactored wallpaper loading from per-window
Picture::for_filename() to shared gdk::Texture pattern (matching
moonset/moongreet), avoiding redundant JPEG decoding on multi-monitor.

src/main.rs

@@ -14,7 +14,6 @@ use gtk4::prelude::*;
 use gtk4::{self as gtk, gio};
 use gtk4_session_lock;
 use std::cell::RefCell;
-use std::path::PathBuf;
 use std::rc::Rc;
 
 use crate::fingerprint::FingerprintListener;
@@ -42,14 +41,15 @@ fn activate(app: &gtk::Application) {
 
     let config = config::load_config(None);
     let bg_path = config::resolve_background_path(&config);
+    let bg_texture = lockscreen::load_background_texture(&bg_path, config.background_blur);
 
     if gtk4_session_lock::is_supported() {
-        activate_with_session_lock(app, &display, &bg_path, &config);
+        activate_with_session_lock(app, &display, &bg_texture, &config);
     } else {
         #[cfg(debug_assertions)]
         {
             log::warn!("ext-session-lock-v1 not supported — running in development mode");
-            activate_without_lock(app, &bg_path, &config);
+            activate_without_lock(app, &bg_texture, &config);
         }
         #[cfg(not(debug_assertions))]
         {
@@ -62,7 +62,7 @@ fn activate(app: &gtk::Application) {
 fn activate_with_session_lock(
     app: &gtk::Application,
     display: &gdk::Display,
-    bg_path: &PathBuf,
+    bg_texture: &gdk::Texture,
     config: &config::Config,
 ) {
     let lock = gtk4_session_lock::Instance::new();
@@ -87,7 +87,7 @@ fn activate_with_session_lock(
             .and_then(|obj| obj.downcast::<gdk::Monitor>().ok())
         {
             let handles = lockscreen::create_lockscreen_window(
-                bg_path,
+                bg_texture,
                 config,
                 app,
                 unlock_callback.clone(),
@@ -144,7 +144,7 @@ fn init_fingerprint_async(all_handles: Vec<lockscreen::LockscreenHandles>) {
 #[cfg(debug_assertions)]
 fn activate_without_lock(
     app: &gtk::Application,
-    bg_path: &PathBuf,
+    bg_texture: &gdk::Texture,
     config: &config::Config,
 ) {
     let app_clone = app.clone();
@@ -153,7 +153,7 @@ fn activate_without_lock(
     });
 
     let handles = lockscreen::create_lockscreen_window(
-        bg_path,
+        bg_texture,
         config,
         app,
         unlock_callback,