fix(gui): use imscope context manager for prior session tint in _gui_func

Convert manual push_style_color / pop_style_color in _gui_func to use the
imscope context manager so the pop is exception-safe via Python's with
statement. Manual push/pop can desync if render_main_interface raises
mid-render, causing 'PopStyleColor() too many times!' imGui assertion
on subsequent frames.

The try/except around render_main_interface was already there but the
pop was outside it, so the pop count could exceed the push count when
an exception short-circuited the render.

src/gui_2.py

@@ -771,32 +771,30 @@ class App:
   render_shader_live_editor(self)
   render_history_window(self)
   pushed_prior_tint = False
-  
+
   # Render background shader
   bg = bg_shader.get_bg()
   ws = imgui.get_io().display_size
   if bg.enabled:
    bg.render(ws.x, ws.y)
-  
+
   theme.render_post_fx(ws.x, ws.y, self.ai_status, self.ui_crt_filter)
-  
+
   if self.perf_profiling_enabled: self.perf_monitor.start_component("_gui_func")
-  if self.is_viewing_prior_session:
-   imgui.push_style_color(imgui.Col_.window_bg, vec4(50, 40, 20))
-   pushed_prior_tint = True
-  
   try:
-   render_main_interface(self)
+   if self.is_viewing_prior_session:
+    with imscope.style_color(imgui.Col_.window_bg, vec4(50, 40, 20)):
+     render_main_interface(self)
+   else:
+    render_main_interface(self)
   except Exception as e:
    sys.stderr.write(f"ERROR in _gui_func: {e}\n")
    traceback.print_exc()
-  
-  if pushed_prior_tint:
-   imgui.pop_style_color()
-  
+
   self._handle_history_logic()
-  
-  if self.perf_profiling_enabled: self.perf_monitor.end_component("_gui_func")
+   
+  if self.perf_profiling_enabled:
+   self.perf_monitor.end_component("_gui_func")
   return
 
  def _render_window_if_open(self, name: str, render_func: Callable[[], None], flag_condition: bool = True) -> None: