fix: proper terminal buffer save/restore with full pyte screen state

Save complete pyte screen state (characters, colors, attributes,
cursor position, modes) via pickle serialization instead of just
plain text. Restore via direct buffer manipulation + full redraw.
Fixes broken/garbled layout when switching between servers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gui/tabs/terminal_tab.py

@@ -86,12 +86,13 @@ class TerminalTab(ctk.CTkFrame):
                         session.rows = rows
                         session.connect()
                     else:
-                        # Reused session — restore saved buffer instead of resetting
+                        # Reused session — restore full screen state from pool
                         saved_buf = self.session_pool.get_shell_state(alias)
                         def _restore_buffer():
-                            self._terminal.reset()
                             if saved_buf:
-                                self._terminal.feed(saved_buf + b"\n")
+                                self._terminal.restore_buffer(saved_buf)
+                            else:
+                                self._terminal.reset()
                         self.after(0, _restore_buffer)
 
                     # Set up callbacks even if session already existed