fix: #3625201 Keep the subscribe block button black; indigo only on the Newsletter hero

The subscribe block (13221:9240) draws a black button; it rendered indigo because newsletter-subscribe.theme.scss keyed the hero's indigo on .bg-accent-subtle, which the block shares.

Now keyed on placement: the hero puts the webform block straight into its column (.row > [class*=col-] > .block-webform); the subscribe block wraps it in a Group row and falls through to the theme's black button.

Verified on both: article block rgb(0,0,0), hero rgb(79,70,225), both 16px/500, 12x24 padding, square.

Closes #3625201.

Checkpoints

• Addition/Change/Update/Fix to this project
• Testing to ensure no regression
• Reviewed by a human
• Code review by maintainers
• Release

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