fix: audit LOW fixes — dead uid, home_dir warn, clippy sweep, debug value (v0.8.5)

- users::User: drop the unused `uid` field and its getuid() assignment.
  The compiler dead_code warning is gone, and the synthetic `u32::MAX`
  sentinel in the panel fallback is obsolete too.
- panel: surface a log::warn! when dirs::home_dir() returns None instead
  of silently falling back to an empty PathBuf that would make avatars
  look for .face in the current working directory.
- Apply three clippy suggestions: two collapsible if-let + && chains in
  users::get_avatar_path_with and config::resolve_background_path_with,
  and a redundant closure in panel::execute_action's spawn_blocking.
- main: require MOONSET_DEBUG=1 to escalate log verbosity — mere
  presence of the var must not dump path info into the journal.

Cargo.lock

@@ -616,7 +616,7 @@ dependencies = [
 
 [[package]]
 name = "moonset"
-version = "0.8.4"
+version = "0.8.5"
 dependencies = [
  "dirs",
  "gdk-pixbuf",