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

Cargo.lock

@@ -569,7 +569,7 @@ dependencies = [
 
 [[package]]
 name = "moongreet"
-version = "0.4.0"
+version = "0.4.1"
 dependencies = [
  "gdk-pixbuf",
  "gdk4",