Add reaction popover, socket guards, dedupe reactions

Build a members lookup and enrich reaction groups with reactor info; introduce a ReactionPill component that shows a tabbed popover listing who reacted (with ability to remove your own reaction). Move reaction button into the new component and adapt styling/UX.

Fix WebSocket handling in useChatSocket by adding a mounted guard to avoid handling events from torn-down sockets (addresses React StrictMode double-invoke issues) and avoid setting state after unmount.

Prevent duplicate local reaction entries in ChatRoomPage by skipping "added" events when the same user/emoji reaction already exists.

frontend/src/pages/social/chat/ChatRoomPage.tsx

@@ -198,7 +198,12 @@ export default function ChatRoomPage() {
                 if (m.id !== event.message_id) return m;
                 let reactions = [...(m.reactions ?? [])] as typeof m.reactions;
                 if (event.action === "added") {
-                  reactions = [...reactions, { id: -Date.now(), user: event.user_id, emoji: event.emoji, created_at: new Date().toISOString() } as never];
+                  const alreadyExists = reactions.some(
+                    (r) => (r.user as unknown as number) === event.user_id && r.emoji === event.emoji,
+                  );
+                  if (!alreadyExists) {
+                    reactions = [...reactions, { id: -Date.now(), user: event.user_id, emoji: event.emoji, created_at: new Date().toISOString() } as never];
+                  }
                 } else if (event.action === "removed") {
                   reactions = reactions.filter((r) => !((r.user as unknown as number) === event.user_id && r.emoji === event.emoji));
                 } else if (event.action === "switched") {