Exit sub-network via Escape key

[Sambhram1]

Problem
Previously, pressing the Escape key while inside a sub-network would close the entire node graph overlay, forcing users to lose their navigation context and start over from the beginning. This was frustrating when working with deeply nested networks.

Solution
Modified the DocumentMessage::Escape handler in document_message_handler.rs to check if the user is currently inside a nested network before closing the graph overlay:

If inside a sub-network (breadcrumb path is not empty): Exit one level up using ExitNestedNetwork { steps_back: 1 }
If at the root level (breadcrumb path is empty): Close the graph overlay as before
Changes
Updated document_message_handler.rs - Modified the Escape message handler to conditionally navigate up one level or close the overlay based on the current navigation depth
Testing
The fix can be tested by:

Opening the node graph
Entering a sub-network (double-clicking a node)
Pressing Escape - should return to the parent network
Pressing Escape again at the root level - should close the graph overlay
Benefits
More intuitive navigation behavior
Preserves user context when working with nested networks
Consistent with common UI patterns where Escape means "go back one step"

Closes #3283

[timon-schelling]

!build

[github-actions]

📦 Build Complete for c29b426
https://5b1eee74.graphite.pages.dev

[timon-schelling]

I would remove the comment, feels like it doesn't add information. Code looks good. I'll test it later.

[Sambhram1]

Ok no issues

[TrueDoctor]

I think the comment is fine and even though it does not provide extra information, it helps with skimming over the code

[timon-schelling]

Also fine with me.

[j-jasz]

This PR should also close: #3214

[timon-schelling]

Exactly what I wanted I to do. Thanks for contributing :)