feat(gui): Unified window state and fixed context preservation regressions

- Implement unified show_windows['Text Viewer'] state and fix docking conflict loops.
- Fix Tool Call row interactivity using spanned selectables.
- Fix context selection loss when switching/creating discussions.
- Implement 'Empty Context Warning' modal for safer generation.
- Correct IndentationError in app_controller.py.
- Remove legacy show_text_viewer attribute and update API hooks.

src/app_controller.py

@@ -927,7 +927,6 @@ class AppController:
   self.models_thread: Optional[threading.Thread] = None
   self.show_windows: Dict[str, bool] = {}
   self.show_script_output: bool = False
-  self.show_text_viewer: bool = False
   self.text_viewer_title: str = ''
   self.text_viewer_content: str = ''
   self.text_viewer_type: str = 'text'
@@ -1064,7 +1063,6 @@ class AppController:
    'ui_separate_tier2': 'ui_separate_tier2',
    'ui_separate_tier3': 'ui_separate_tier3',
    'ui_separate_tier4': 'ui_separate_tier4',
-   'show_text_viewer': 'show_text_viewer',
    'text_viewer_title': 'text_viewer_title',
    'text_viewer_type': 'text_viewer_type',
    'disc_entries': 'disc_entries',
@@ -1122,7 +1120,6 @@ class AppController:
    'ui_separate_tier2': 'ui_separate_tier2',
    'ui_separate_tier3': 'ui_separate_tier3',
    'ui_separate_tier4': 'ui_separate_tier4',
-   'show_text_viewer': 'show_text_viewer',
    'text_viewer_title': 'text_viewer_title',
    'text_viewer_type': 'text_viewer_type'
   })
@@ -1718,6 +1715,7 @@ class AppController:
    "Message": False,
    "Response": False,
    "Tool Calls": False,
+   "Text Viewer": False,
    "Theme": True,
    "Log Management": False,
   }