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.
- 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
