fix: audit LOW fixes — stdout null, utf-8 path, debug value, hidden sessions (v0.8.6)

- power::run_command: .stdout(Stdio::null()) — the pipe was never drained,
  structurally fragile even if no current caller hits it.
- config: replace to_string_lossy() on relative wallpaper paths with
  to_str() + log::warn, so non-UTF-8 paths are dropped cleanly instead
  of being mangled into unopenable U+FFFD strings.
- main: require MOONGREET_DEBUG=1 to raise verbosity. Mere presence of
  the var must not leak socket paths, usernames, and auth round counts
  into the journal.
- sessions: parse Hidden= and NoDisplay= keys, skip entries marked true.
  Keeps disabled or stub .desktop files out of the session dropdown.

src/sessions.rs

@@ -23,6 +23,8 @@ fn parse_desktop_file(path: &Path, session_type: &str) -> Option<Session> {
     let mut in_section = false;
     let mut name: Option<String> = None;
     let mut exec_cmd: Option<String> = None;
+    let mut hidden = false;
+    let mut no_display = false;
 
     for line in content.lines() {
         let line = line.trim();
@@ -44,9 +46,18 @@ fn parse_desktop_file(path: &Path, session_type: &str) -> Option<Session> {
             && exec_cmd.is_none()
         {
             exec_cmd = Some(value.to_string());
+        } else if let Some(value) = line.strip_prefix("Hidden=") {
+            hidden = value.eq_ignore_ascii_case("true");
+        } else if let Some(value) = line.strip_prefix("NoDisplay=") {
+            no_display = value.eq_ignore_ascii_case("true");
         }
     }
 
+    if hidden || no_display {
+        log::debug!("Skipping {}: Hidden/NoDisplay entry", path.display());
+        return None;
+    }
+
     let name = name.filter(|s| !s.is_empty());
     let exec_cmd = exec_cmd.filter(|s| !s.is_empty());