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/ipc.rs

@@ -84,6 +84,9 @@ pub fn send_message(
         return Err(IpcError::PayloadTooLarge(payload.len()));
     }
 
+    let msg_type = msg.get("type").and_then(|v| v.as_str()).unwrap_or("unknown");
+    log::debug!("IPC send: type={msg_type}, size={} bytes", payload.len());
+
     let header = (payload.len() as u32).to_le_bytes();
     stream.write_all(&header)?;
     stream.write_all(&payload)?;
@@ -101,6 +104,8 @@ pub fn recv_message(stream: &mut UnixStream) -> Result<serde_json::Value, IpcErr
 
     let payload = recv_payload(stream, length)?;
     let value: serde_json::Value = serde_json::from_slice(&payload)?;
+    let msg_type = value.get("type").and_then(|v| v.as_str()).unwrap_or("unknown");
+    log::debug!("IPC recv: type={msg_type}, size={length} bytes");
     Ok(value)
 }