feat(messaging-ui): reaction picker + attachments in the Thread

- useMessages exposes upload(); Thread gains a hover reaction picker (react to
  a message) and an attach button (upload → stage → send), plus inline image /
  file rendering of message attachments — all capability-degrading (hidden when
  the adapter lacks react/upload)
- themeable styles for the picker, attach button, staged chip, and attachments
- 57 tests still green

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

packages/iios-messaging-ui/src/hooks/use-messages.ts

@@ -1,7 +1,7 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
 import { useAdapter } from '../provider';
 import { isOwnMessage } from '../types';
-import type { Message, SendOpts } from '../types';
+import type { Attachment, Message, SendOpts } from '../types';
 
 const TYPING_TTL_MS = 3500;
 
@@ -15,6 +15,7 @@ export interface MessagesState {
   error: string | null;
   send: (content: string, opts?: SendOpts) => Promise<void>;
   react: (messageId: string, emoji: string) => Promise<void>;
+  upload: (file: File) => Promise<Attachment>;
   typingUserIds: string[];
   seenIds: Set<string>;
   sendTyping: () => void;
@@ -147,6 +148,14 @@ export function useMessages(threadId: string | null): MessagesState {
     [adapter, threadId],
   );
 
+  const upload = useCallback(
+    async (file: File): Promise<Attachment> => {
+      if (!adapter.upload) throw new Error('uploads are not supported by this adapter');
+      return adapter.upload(file);
+    },
+    [adapter],
+  );
+
   const sendTyping = useCallback(() => {
     if (threadId) adapter.sendTyping(threadId);
   }, [adapter, threadId]);
@@ -196,6 +205,7 @@ export function useMessages(threadId: string | null): MessagesState {
     error,
     send,
     react,
+    upload,
     typingUserIds,
     seenIds: seenMine,
     sendTyping,