Feature request: Add chapter-level checkbox to toggle all scene selections in Context Panel

[burbilog]

Description:
Selecting scenes one-by-one in the Context Panel is tedious for large projects. Please add a checkbox on each chapter that selects/deselects all of its scenes in one action.

Use case:

• With many chapters and scenes, re-selecting each scene individually is time-consuming and error-prone when configuring LLM context.

Proposed solution:

• Add a checkbox on the chapter row.
• Checking the chapter checkbox selects all scenes under that chapter.
• Unchecking it deselects all scenes.
• If only some scenes are selected, show the chapter checkbox in an indeterminate (partial) state.
• Toggling any scene updates the chapter’s checkbox state accordingly.

Acceptance criteria:

• Each chapter has a parent checkbox controlling all of its child scene checkboxes.
• Parent checkbox supports three states: checked, unchecked, indeterminate.
• Child and parent checkbox states stay in sync as selections change.
• Behavior works consistently after expanding/collapsing chapters and reopening the panel.

Additional context:

• This complements the existing context selection workflow and reduces click fatigue for large outlines.

[stump-wiz]

I'll work on adding this. I'm concerned that including all the scenes in a prompt will overflow the LLM's prompt window, which is why I added Chapter Summaries to the Context Panel. Since LLM's are able to handle larger context, it seems reasonable to add this capability.

If the LLM rejects the prompt because it is too long, Writingway will just pass that back to the user like it does today.