fix(web-components): prevent dialog content shift on overflow

[sylvesterkaczmarek]

Summary of the changes

Fixes IcDialog content shifting horizontally when the content area becomes vertically scrollable.

The dialog content uses overflow-y: auto, so when a vertical scrollbar appears the available content width can shrink by the scrollbar width. This is visible when dynamic content expands, for example accordions or open selects.

This change reserves the vertical scrollbar gutter on the dialog content area using scrollbar-gutter: stable, keeping the content box width stable before and after overflow appears.

No public API or visual design changes.

Related issue

Closes #3350

Testing

• Adds a React Cypress regression that measures the content-area width before overflow.
• Dynamically adds content until vertical overflow is present.
• Verifies the content-area width remains unchanged.
• Production change is one CSS declaration.
• Production and React regression changes are split by package scope.
• Branch is based directly on develop.

Full upstream CI will run when the PR is opened.

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1 out of 2 committers have signed the CLA.

✅ GCHQ-Developer-530
❌ Sylvester Kaczmarek

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