fix(messaging-ui): composer height was being clobbered by a class-name collision

The chat composer reused .miu-textarea, which the INBOX MAIL composer already
owned further down the stylesheet with `resize: vertical; min-height: 90px`.
Equal specificity, later rule wins — so the mail styling applied to the chat box:
90px tall with a resize grabber, and every height fix in 0.1.10–0.1.12 was
silently overridden. That is why the box never changed.

The composer now uses its own .miu-composer-box; the inbox rule is untouched.
Adds a regression test asserting the composer does not carry .miu-textarea.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/components/composer.tsx

@@ -203,7 +203,7 @@ export function Composer({
         ) : null}
         <textarea
           ref={inputRef}
-          className="miu-input miu-textarea"
+          className="miu-input miu-composer-box"
           value={draft}
           placeholder={placeholder}
           aria-label="Message"