feat: add sub-progress bar for rsync file copy progress

- InstallMessage: added SubProgress(f32) and CurrentFile(String) variants
- GlobalState: added install_sub_progress field
- UiSender: added sub_progress() and current_file() helpers
- CopyFilesJob: now streams rsync stderr, parses xfr#/to-chk for progress,
  and sends SubProgress/CurrentFile messages to UI
- execution.rs: displays secondary progress bar below the main one during
  rsync file copy, updates install_current_file in real-time
- main.rs: handles new InstallMessage variants in auto-install mode

src/main.rs

@@ -188,6 +188,8 @@ fn run_auto_install(path: &str, demo_mode: bool) -> eframe::Result<()> {
                     print!("\r  {}: [{:50}] {:.0}%", t!("progress"), "=".repeat((p * 50.0) as usize), p * 100.0);
                     let _ = std::io::Write::flush(&mut std::io::stdout());
                 }
+                Ok(InstallMessage::SubProgress(_)) => {}
+                Ok(InstallMessage::CurrentFile(_)) => {}
                 Ok(InstallMessage::Done) => {
                     println!("\n\n{}: {}", t!("installation_done"), t!("installation_done_description"));
                     break;