fix(gui): Resolve ImGui stack corruption, JSON serialization errors, and test regressions

tests/test_mma_approval_indicators.py

@@ -5,7 +5,7 @@ from src.gui_2 import App
 
 
 def _make_app(**kwargs):
- app = MagicMock(spec=App)
+ app = MagicMock()
  app.mma_streams = kwargs.get("mma_streams", {})
  app.mma_tier_usage = kwargs.get("mma_tier_usage", {
   "Tier 1": {"input": 0, "output": 0, "model": "gemini-3.1-pro-preview"},
@@ -13,6 +13,7 @@ def _make_app(**kwargs):
   "Tier 3": {"input": 0, "output": 0, "model": "gemini-2.5-flash-lite"},
   "Tier 4": {"input": 0, "output": 0, "model": "gemini-2.5-flash-lite"},
  })
+ app.ui_focus_agent = kwargs.get("ui_focus_agent", None)
  app.tracks = kwargs.get("tracks", [])
  app.active_track = kwargs.get("active_track", None)
  app.active_tickets = kwargs.get("active_tickets", [])