fix: Conform standalone button width to its container #2361

[marek-mihok]

The PR fulfills these requirements: (check all the apply)

• It's submitted to the main branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
• When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
• If changes were made to ui folder, unit tests (make test) still pass.
• New/updated tests are included

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The idea of this fix is to make the standalone button together with its left margin to fit its container width. This is by specifying display: 'flex for the button wrapper.

Example:
When the standalone button with has specified width of 150px is followed by another standalone button with the same width, the first button will be 150px wide but the second one and every other will be 142px wide as it accounts for 8px left padding. This marginal difference can only be caught by a perfectionist eye. This is demonstrated in the recording below:

Screen.Recording.2024-09-12.at.08.46.16.mov

The alignItems: 'center' is specified not to break existing button stylings.

The solution was manually and visual regression tested.

Closes #2361

[mturoci]

LGTM, thanks @marek-mihok!