restore/undo for deleted messages — backend + UI

- POST /api/channels/{id}/messages/{msg_id}/restore — undeletes a message
- Any user can restore (not just the deleter)
- Broadcasts restored message via WebSocket
- UI: "undo" button in floating pill on deleted messages
- API: restoreMessage() in api.ts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/colony/src/App.tsx

@@ -1,7 +1,7 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import type { Channel } from "@/types/Channel";
 import type { Message } from "@/types/Message";
-import { getChannels, getMessages, getCurrentUsername, deleteMessage } from "@/api";
+import { getChannels, getMessages, getCurrentUsername, deleteMessage, restoreMessage } from "@/api";
 import { ChannelSidebar } from "@/components/ChannelSidebar";
 import { MessageItem } from "@/components/MessageItem";
 import { ComposeBox } from "@/components/ComposeBox";
@@ -251,6 +251,14 @@ export default function App() {
                         // ignore
                       }
                     }}
+                    onRestore={async (chId, msgId) => {
+                      try {
+                        await restoreMessage(chId, msgId);
+                        loadMessages();
+                      } catch {
+                        // ignore
+                      }
+                    }}
                   />
                 </div>
               );