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

Makefile.am

@@ -1,2 +1,10 @@
 SUBDIRS = src
-EXTRA_DIST = README.md
+EXTRA_DIST = README.md desktop/x3-ssh.desktop icons/x3-ssh.png
+
+# Install desktop file to system applications directory
+applicationsdir = $(datadir)/applications
+dist_applications_DATA = desktop/x3-ssh.desktop
+
+# Install icon to system hicolor theme
+iconsdir = $(datadir)/icons/hicolor/256x256/apps
+dist_icons_DATA = icons/x3-ssh.png