Email field and Subscribe button still differ in height on themes with taller buttons

Problem/Motivation

#3622822 gave the email input and the Subscribe button the same explicit minimum height, which fixed the mismatch on Horizon Aid. It does not fully fix it on a theme whose button computes taller than that floor.

Measured on Educare, at 1440px, with that change in place:

email input   56px   (the min-height floor)
Subscribe     58px   (theme button padding)

A 2px mismatch rather than 15px, so much better than before, but the two controls still do not line up. The floor cannot fix this on its own: it is a minimum, and any theme whose button lands above it reopens the gap by exactly that difference.

Steps to reproduce

1. Install a site on a theme whose button padding exceeds 3.5rem, for example Educare.
2. Visit a page carrying the Newsletter Subscribe form.
3. Measure the email input and the Subscribe button.

Proposed resolution

Add h-100 to the email input, alongside the minimum height:

'#attributes':
  class:
    - w-100
    - h-100
    - rounded-pill
    - px-4
  style: 'min-height:3.5rem'

The two work together and each covers what the other cannot. On a wide screen the row is a flex row with align-items-stretch, so the wrapper is already stretched to the button height and h-100 makes the input fill it, whatever that height is. On a narrow screen the row becomes a column, the cross axis is width, and h-100 does nothing, so the minimum height keeps the input above the 44px touch target.

Using h-100 alone was tried first and rejected in #3622822 precisely because it fails on mobile. The floor alone is what this issue fixes.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ✅ UX/UI designer responsibilities
• ✅ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release varbase_webform_base-1.0.2

User interface changes

• The email input matches the Subscribe button height on any theme, not only where the button happens to equal the floor.

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• The Newsletter Subscribe email field now matches the Subscribe button height on every theme.