fix: restrict textarea resize to vertical and enable auto-grow

[Aswath-Senthilkumar]

Done

• Restricted textarea resize to vertical only (resize: vertical)
• Enabled auto-grow in height as the user types (field-sizing: content)
• Added configurable $textarea-min-height (defaults to 3 lines) and $textarea-max-height (defaults to none) variables in _settings_spacing.scss with !default flag for user override
• Updated docs heading with {{ status('updated') }} label
• Added entry to releases.yml
• field-sizing: content has ~80% global browser support as of March 2026 ([caniuse]). Unsupported browsers fall back gracefully, the textarea remains functional with fixed sizing and vertical-only resize still applies.

QA

• Verify textarea only resizes vertically, the horizontal drag handle should be gone
• Verify textarea grows in height automatically as text is typed
• Verify textarea starts at a 3-row minimum height
• Tested locally using dotrun on WSL with the docs site at localhost:8101

Screenshots

Resizing:

Before:

After:

Auto-grow:

Before:

After:

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Fixes #5713

[webteam-app]

Aswath-Senthilkumar is not a collaborator of the repo

[Aswath-Senthilkumar]

Should $textarea-min-height and $textarea-max-height be documented on the spacing settings page? I noticed the existing form input variables ($input-margin-bottom, $input-border-thickness) aren't listed there either, so I wasn't sure if that's the right place. Happy to add it in this PR or as a follow-up if needed.