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/styles.css

@@ -524,7 +524,9 @@ button.miu-attach-dl:hover {
 /* Chat box: one row by default, grows with content (JS sets height), then scrolls.
    border-box so min/max-height are the REAL height — under the default content-box the padding
    and border are added on top, which made a "38px" box render ~58px and dragged the row with it. */
-.miu-textarea {
+/* Composer box only. NOTE: .miu-textarea is already taken by the inbox mail composer further
+   down (resize: vertical; min-height: 90px) — sharing it made that rule win and blow this up. */
+.miu-composer-box {
   box-sizing: border-box;
   resize: none;
   /* Collapsed height must equal the single-line <input> this replaced: 20px of line + 16px