Simpler approach: disable the view picker toggle when preview is running,
preventing users from switching views during active runs.
This replaces the previous mounted ref guard approach (commits a0188bd9b5,
b7f1eb9b7b, 8332f0de2b) which added complexity to handle post-unmount
operations. Disabling the toggle is more direct and follows KISS principle.
Changes:
- Add disabled prop to ViewPicker based on isResponding state
- Revert mounted ref guards in use-chat-flow-control.ts
- Revert isMountedRef parameter in use-nodes/edges-interactions-without-sync.ts
- Revert defensive type check in markdown-utils.ts (no longer needed)
Related to a9c5201485 - when switching views during active preview run,
the markdown preprocessors could receive non-string content (e.g., frozen
arrays from immer). Returning the original value caused ReactMarkdown to
fail with "Cannot assign to read only property" error.
Now both preprocessLaTeX and preprocessThinkTag return '' for non-string
input, preventing runtime errors during view switches.
