fix: address audit findings — polling, symlinks, validation, wallpaper removal (v0.7.0)

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)

Cargo.lock

@@ -616,7 +616,7 @@ dependencies = [
 
 [[package]]
 name = "moonset"
-version = "0.5.0"
+version = "0.7.0"
 dependencies = [
  "dirs",
  "gdk-pixbuf",