Three parallel audits (quality, performance, security) identified issues
across the codebase. This commit addresses all remaining findings:
- Replace busy-loop polling in run_command with child.wait() + timeout thread
- Canonicalize ~/.face and AccountsService avatar paths to prevent symlink abuse
- Add detect_locale_with() DI function for testable locale detection
- Move config I/O from activate() to main() to avoid blocking GTK main loop
- Validate background_blur range (0–200), reject invalid values with warning
- Remove embedded wallpaper from GResource — moonarch provides it via filesystem
(binary size ~3.2MB → ~1.3MB)