fix: critical bugs + .desktop file + system install

Bug fixes:
- Fixed PTY bridge: IO thread now uses pty_slave_ instead of pty_master_
  (remote output was never reaching the terminal)
- Fixed double-delete crash on tab close by removing page-removed callback
- Fixed port being ignored in password dialog (always used port 22)
- Added EINTR handling in select() for IO thread
- Added mutex for thread-safe cleanup
- Added try/catch for std::stoi in server config parsing
- VtePty lifecycle now managed properly (no double-close)
- Redundant ssh_channel_poll_timeout call removed

New:
- Added .desktop file for system-wide application menu entry
- Makefile.am installs desktop file to $(datadir)/applications
- Makefile.am installs icon to $(datadir)/icons/hicolor/256x256/apps

desktop/x3-ssh.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=x3-ssh
+GenericName=SSH Client
+Comment=A GTK3 SSH client with tabbed terminal, server management and key storage
+Exec=x3-ssh
+Icon=x3-ssh
+Terminal=false
+Categories=Network;RemoteAccess;SSH;
+Keywords=ssh;remote;terminal;shell;secure;
+StartupNotify=true