feat(messaging-ui): Slack-style threaded-replies side panel

Replies (messages with parentInteractionId) now open in a right-hand ThreadPane
instead of inline quoting:
- extracted a shared Composer (draft + @mention autocomplete + attach) and
  MessageItem (bubble + mentions + attachment + reactions + reply affordance)
- Thread shows top-level messages only; a message with replies gets a
  '💬 N replies' link, and hovering shows 💬 'Reply in thread'
- ThreadPane renders the root + its replies + a composer that posts back with
  parentInteractionId; Messenger becomes 3-column (list | thread | pane),
  pane closes on conversation switch
- no contract/adapter/backend change (parentInteractionId was already wired)
- thread-pane render test (open → reply → parent shows the count); 58 green

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

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

@@ -0,0 +1,147 @@
+import { useMemo, useRef, useState, type ChangeEvent, type FormEvent, type KeyboardEvent } from 'react';
+import {
+  SPECIAL_MENTIONS,
+  insertMention,
+  resolveMentions,
+  trailingMentionQuery,
+} from '../mentions';
+import type { Attachment, Person, SendOpts } from '../types';
+
+interface Suggestion {
+  key: string;
+  label: string;
+  insert: string;
+}
+
+/**
+ * The message input: draft, @mention autocomplete, and attachment staging. Shared by the main
+ * Thread and the ThreadPane (which passes a parentInteractionId so a reply lands in the thread).
+ */
+export function Composer({
+  members,
+  canUpload,
+  upload,
+  onSend,
+  onTyping,
+  parentInteractionId,
+  placeholder = 'Type a message… @ to mention',
+}: {
+  members: Person[];
+  canUpload: boolean;
+  upload: (file: File) => Promise<Attachment>;
+  onSend: (text: string, opts?: SendOpts) => Promise<void>;
+  onTyping?: () => void;
+  parentInteractionId?: string;
+  placeholder?: string;
+}) {
+  const [draft, setDraft] = useState('');
+  const [sending, setSending] = useState(false);
+  const [staged, setStaged] = useState<Attachment | null>(null);
+  const [uploading, setUploading] = useState(false);
+  const fileRef = useRef<HTMLInputElement>(null);
+
+  const query = trailingMentionQuery(draft);
+  const suggestions = useMemo<Suggestion[]>(() => {
+    if (query === null) return [];
+    const q = query.toLowerCase();
+    const specials = SPECIAL_MENTIONS.filter((s) => s.startsWith(q)).map((s) => ({ key: `@${s}`, label: `@${s}`, insert: s }));
+    const people = members.filter((m) => m.name.toLowerCase().includes(q)).map((m) => ({ key: m.id, label: m.name, insert: m.name }));
+    return [...specials, ...people].slice(0, 6);
+  }, [query, members]);
+  const showSuggest = query !== null && suggestions.length > 0;
+
+  function pick(insert: string): void {
+    setDraft((d) => insertMention(d, insert));
+  }
+
+  async function onPickFile(e: ChangeEvent<HTMLInputElement>): Promise<void> {
+    const file = e.target.files?.[0];
+    e.target.value = '';
+    if (!file) return;
+    setUploading(true);
+    try {
+      setStaged(await upload(file));
+    } catch {
+      /* host surfaces upload errors */
+    } finally {
+      setUploading(false);
+    }
+  }
+
+  async function submit(e?: FormEvent): Promise<void> {
+    e?.preventDefault();
+    const text = draft.trim();
+    if ((!text && !staged) || sending) return;
+    const mentions = resolveMentions(text, members);
+    const att = staged;
+    setDraft('');
+    setStaged(null);
+    setSending(true);
+    try {
+      await onSend(text, {
+        ...(mentions.length ? { mentions } : {}),
+        ...(att ? { attachment: att } : {}),
+        ...(parentInteractionId ? { parentInteractionId } : {}),
+      });
+    } catch {
+      setStaged(att);
+    } finally {
+      setSending(false);
+    }
+  }
+
+  function onKeyDown(e: KeyboardEvent<HTMLInputElement>): void {
+    if (showSuggest && e.key === 'Enter') {
+      e.preventDefault();
+      pick(suggestions[0]!.insert);
+    }
+  }
+
+  return (
+    <form className="miu-composer" onSubmit={submit}>
+      {showSuggest ? (
+        <ul className="miu-suggest" role="listbox" aria-label="Mention suggestions">
+          {suggestions.map((s) => (
+            <li key={s.key}>
+              <button type="button" role="option" aria-selected="false" className="miu-suggest-item" onClick={() => pick(s.insert)}>
+                {s.label}
+              </button>
+            </li>
+          ))}
+        </ul>
+      ) : null}
+      {staged ? (
+        <div className="miu-staged">
+          📎 {staged.name}
+          <button type="button" className="miu-staged-x" onClick={() => setStaged(null)} aria-label="Remove attachment">
+            ✕
+          </button>
+        </div>
+      ) : null}
+      <div className="miu-composer-row">
+        {canUpload ? (
+          <>
+            <input ref={fileRef} type="file" className="miu-file-input" onChange={onPickFile} aria-label="Attach a file" />
+            <button type="button" className="miu-attach-btn" title="Attach a file" disabled={uploading} onClick={() => fileRef.current?.click()}>
+              {uploading ? '…' : '📎'}
+            </button>
+          </>
+        ) : null}
+        <input
+          className="miu-input"
+          value={draft}
+          placeholder={placeholder}
+          aria-label="Message"
+          onChange={(e) => {
+            setDraft(e.target.value);
+            onTyping?.();
+          }}
+          onKeyDown={onKeyDown}
+        />
+        <button type="submit" className="miu-send" disabled={(!draft.trim() && !staged) || sending}>
+          Send
+        </button>
+      </div>
+    </form>
+  );
+}