Move the textarea's rules out of the field into textarea.css

<x-textarea> marks its field root data-md-textarea, and what a
<textarea> changes in the field — the grip in the corner, the label on
the first line, autogrow between rows and max-rows, the filled offsets —
moves from field.css to components/textarea.css (plan step 36).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/css/components.css

@@ -18,6 +18,7 @@
 @import './components/field.css';
 @import './components/input.css';
 @import './components/password.css';
+@import './components/textarea.css';
 
 /* Containment */