[Feat]: Stretch canvas height for fixed-row layout

[NobleRaccoon]

Proposed changes

Add a stretch mode for canvas height in fixed-row layouts and expose it through a new canvas setting.

This is intended to support responsive canvas layouts where the editor can fill the available viewport height instead of staying constrained to a fixed canvas height.

This change:

• adds a new gridStretchHeight app setting for canvas height stretching
• keeps the runtime behavior constrained to read-only canvas rendering
• extracts the stretch decision into small pure helpers
• adds unit coverage for the new behavior

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

This change is intentionally limited to the canvas path for now, but it can be a starting point toward unifying row and column sizing behavior, and eventually the container model, under a shared fixed / grow / stretch approach.