test: update grid overlay rendering behavior based on enabled state

[IvanCCO]

• Modified tests to ensure the grid overlay does not render when disabled by default.
• Updated the behavior to render the overlay when the enabled state is set to true.
• Introduced a new function to merge session settings, improving the handling of grid settings based on site state.
• Adjusted default settings to set 'enabled' to false, aligning with the new behavior.

These changes enhance the clarity of the grid's visibility logic and ensure consistent behavior across different states.

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Summary by cubic

Grid overlay is now off by default and only renders when enabled: true. Session UI state is merged into patterns to keep visibility and toolbar position consistent; tests updated to reflect the new behavior.

• Refactors

  • Added mergeSessionSettings to apply session fields (enabled, visible, toolbarX, toolbarY) to pattern settings.
  • Updated getActivePattern, ensureAxisPattern, updateActivePatternSettings, and applyPatternSelection to preserve session state instead of resetting it.

• Migration

  • Explicitly set enabled: true when you want the overlay rendered (e.g., in tests or when sending settings messages).

^{Written for commit a4ffd32. Summary will update on new commits.}