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

DECISIONS.md

@@ -0,0 +1,7 @@
+# Decisions
+
+## 2026-03-28 – Audit fixes for shared wallpaper texture (v0.4.1)
+- **Who**: Selene, Dominik
+- **Why**: Quality, performance, and security audits flagged issues in `load_background_texture()`, debug logging, and greetd error handling
+- **Tradeoffs**: GResource path now requires UTF-8 (returns `None` for non-UTF-8 instead of aborting); 50 MB wallpaper limit is generous but prevents OOM; debug logging off by default trades observability for security
+- **How**: GResource branch via `resources_lookup_data()` + `from_bytes()` (no abort), file size limit, error details only at debug level, `MOONGREET_DEBUG` env var for log level, greetd retry path truncation matching `show_greetd_error()`