feat: rewrite terminal to use vte_terminal_spawn_sync

Replaced the SSH library + IO thread + intermediate PTY bridge with
vte_terminal_spawn_sync(). This eliminates the broken terminal behavior
(backspace, Ctrl+C, copy/paste, bash completion) caused by the
intermediate PTY with no process on the slave side.

- SshTerminal now spawns 'ssh' as a VTE subprocess
- Removed libssh dependency from terminal (still used for key validation)
- Removed IO thread, manual PTY creation, and thread synchronization
- VTE now handles all terminal I/O, line discipline, resize, etc.
- Password saved: uses sshpass -e with SSHPASS env var if available
- Password not saved: ssh prompts interactively in the terminal
- Removed show_password_dialog (no longer needed)
- Simplified connect flow in ssh_client.cpp

src/Makefile.in

@@ -107,8 +107,7 @@ am_x3_ssh_OBJECTS = x3_ssh-main.$(OBJEXT) \
 	x3_ssh-ssh_client.$(OBJEXT) x3_ssh-key_store.$(OBJEXT)
 x3_ssh_OBJECTS = $(am_x3_ssh_OBJECTS)
 am__DEPENDENCIES_1 =
-x3_ssh_DEPENDENCIES = $(am__DEPENDENCIES_1) $(am__DEPENDENCIES_1) \
-	$(am__DEPENDENCIES_1)
+x3_ssh_DEPENDENCIES = $(am__DEPENDENCIES_1) $(am__DEPENDENCIES_1)
 x3_ssh_LINK = $(CXXLD) $(x3_ssh_CXXFLAGS) $(CXXFLAGS) $(AM_LDFLAGS) \
 	$(LDFLAGS) -o $@
 AM_V_P = $(am__v_P_@AM_V@)
@@ -234,8 +233,6 @@ INSTALL_STRIP_PROGRAM = @INSTALL_STRIP_PROGRAM@
 LDFLAGS = @LDFLAGS@
 LIBOBJS = @LIBOBJS@
 LIBS = @LIBS@
-LIBSSH_CFLAGS = @LIBSSH_CFLAGS@
-LIBSSH_LIBS = @LIBSSH_LIBS@
 LTLIBOBJS = @LTLIBOBJS@
 MAKEINFO = @MAKEINFO@
 MKDIR_P = @MKDIR_P@
@@ -313,14 +310,12 @@ x3_ssh_CXXFLAGS = \
 	-std=c++17 \
 	$(GTK_CFLAGS) \
 	$(VTE_CFLAGS) \
-	$(LIBSSH_CFLAGS) \
 	-DLOCALEDIR=\"$(localedir)\" \
 	-DICONDIR=\"$(datadir)/icons\"
 
 x3_ssh_LDADD = \
 	$(GTK_LIBS) \
-	$(VTE_LIBS) \
-	$(LIBSSH_LIBS)
+	$(VTE_LIBS)
 
 iconsdir = $(datadir)/icons
 dist_icons_DATA = ../icons/x3-ssh.png