[WC-3215]: fix(atlas-core): use --suggested-height CSS variable for textarea auto-grow [v3]

[r0b1n]

Problem

The auto-grow textarea library controls height via an inline height: <value>!important style, which prevents customers from overriding textarea height in their own stylesheets.

Solution

The library is patched (via npm patch) to set a CSS custom property instead:

// before
node.style.setProperty('height', height + 'px', 'important');
// after
node.style.setProperty('--suggested-height', height + 'px');

This PR adds the Atlas CSS side of the fix:

.mx-textarea textarea.form-control {
    min-height: var(--suggested-height);
}

Why this approach

• min-height not height — preserves grow-automatically behavior; the textarea can still expand beyond the suggested height.
• Scoped to .mx-textarea — only affects the Mendix textarea widget, not any other widget using textarea.form-control.
• Backwards compatible — when --suggested-height is not set (projects without the library patch), min-height resolves to its initial value and has no effect.
• Customer override — customers can set min-height: 0 or any value on .mx-textarea textarea to fully opt out.

Backport of #336 to lts/mx/9.24 (Atlas 3).

Test plan

• Textarea with Grow automatically = ON: verify it still grows as user types
• Textarea with Grow automatically = OFF: verify it renders at the configured row height
• Customer override: verify setting min-height in custom CSS takes effect