fix: audit findings — wallpaper safety, log filtering, error truncation (v0.4.1)

- Rework load_background_texture(): use resources_lookup_data()/from_bytes()
  for GResource path (no abort on missing resource), add 50 MB file size limit,
  handle non-UTF-8 paths gracefully
- Filter error details to debug level only — warn! logs without internal details
  to prevent system info leaking into journal
- Make debug logging opt-in via MOONGREET_DEBUG env var (default: Info)
- Truncate greetd error description in stale-session retry path using
  MAX_GREETD_ERROR_LENGTH (matching show_greetd_error())
- Add 3 unit tests for load_background_texture edge cases

src/main.rs

@@ -54,18 +54,26 @@ fn activate(app: &gtk::Application) {
     let bg_path = config::resolve_background_path(&config);
     log::debug!("Background path: {}", bg_path.display());
 
+    // Load background texture once — shared across all windows
+    let bg_texture = greeter::load_background_texture(&bg_path);
+    if bg_texture.is_none() {
+        log::error!("Failed to load background texture — greeter will start without wallpaper");
+    }
+
     let use_layer_shell = std::env::var("MOONGREET_NO_LAYER_SHELL").is_err();
     log::debug!("Layer shell: {use_layer_shell}");
 
     // Main greeter window (login UI) — compositor picks focused monitor
-    let greeter_window = greeter::create_greeter_window(&bg_path, &config, app);
+    let greeter_window = greeter::create_greeter_window(bg_texture.as_ref(), &config, app);
     if use_layer_shell {
         setup_layer_shell(&greeter_window, true, gtk4_layer_shell::Layer::Top);
     }
     greeter_window.present();
 
     // Wallpaper-only windows on all monitors (only with layer shell)
-    if use_layer_shell {
+    if use_layer_shell
+        && let Some(ref texture) = bg_texture
+    {
         let monitors = display.monitors();
         log::debug!("Monitor count: {}", monitors.n_items());
         for i in 0..monitors.n_items() {
@@ -73,7 +81,7 @@ fn activate(app: &gtk::Application) {
                 .item(i)
                 .and_then(|obj| obj.downcast::<gdk::Monitor>().ok())
             {
-                let wallpaper = greeter::create_wallpaper_window(&bg_path, app);
+                let wallpaper = greeter::create_wallpaper_window(texture, app);
                 setup_layer_shell(&wallpaper, false, gtk4_layer_shell::Layer::Bottom);
                 wallpaper.set_monitor(Some(&monitor));
                 wallpaper.present();
@@ -87,7 +95,12 @@ fn setup_logging() {
         .unwrap()
         .install()
         .unwrap();
-    log::set_max_level(log::LevelFilter::Debug);
+    let level = if std::env::var("MOONGREET_DEBUG").is_ok() {
+        log::LevelFilter::Debug
+    } else {
+        log::LevelFilter::Info
+    };
+    log::set_max_level(level);
 }
 
 fn main() {