feat: switch to systemd-journal-logger, add debug logging (v0.4.0)

Replace env_logger file-based logging with systemd-journal-logger for
consistency with moonlock and native journalctl integration. Add debug-level
logging at all decision points: config loading, user/session detection,
avatar resolution, locale detection, IPC messages, login flow, and
persistence. No credentials are ever logged.

src/i18n.rs

@@ -127,10 +127,15 @@ pub fn detect_locale() -> String {
         .filter(|s| !s.is_empty())
         .or_else(|| read_lang_from_conf(Path::new(DEFAULT_LOCALE_CONF)));
 
-    match lang {
-        Some(l) => parse_lang_prefix(&l),
+    let result = match lang {
+        Some(ref l) => parse_lang_prefix(l),
         None => "en".to_string(),
-    }
+    };
+    log::debug!("Detected locale: {result} (source: {})", match lang {
+        Some(_) => "LANG env or locale.conf",
+        None => "default",
+    });
+    result
 }
 
 /// Return the string table for the given locale, defaulting to English.