fix(frontend): toggle hide-region class on main canvas

[bobbai00]

What changes were proposed in this PR?

Makes toggleRegion() in menu.component.ts toggle the hide-region CSS class on the main paper element, in addition to the existing body/visibility attribute flip. Follows the same pattern as the neighbouring toggleNumWorkers() / toggleStatus() toggles.

The main editor permanently carries hide-region, whose SCSS rule display: nones every .region child, so the existing body/visibility attribute change had no effect on the main canvas — only on the mini-map (which doesn't carry the class). With this change, the class is added/removed in sync with showRegion, and the existing attribute flip continues to drive the mini-map.

Any related issues, documentation, discussions?

Closes #5120

Regression from #4495.

How was this PR tested?

Not manually browser-tested by the author yet — would appreciate a reviewer running the repro from #5120:

1. Open a workflow with at least two connected operators.
2. Run it so the backend emits region updates.
3. Toggle View → Regions on — hulls should now appear on the main canvas (previous behaviour: only on the mini-map).
4. Toggle off — hulls should disappear on both surfaces.

No unit tests added: toggleRegion() mutates DOM/JointJS state and the surrounding toggles (toggleNumWorkers, toggleStatus) are similarly untested.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Opus 4.7 (1M context)