feat: server members API + member list in sidebar + fix chat input

Backend:
- Add GET /api/v1/servers/{serverID}/members endpoint
- Wire MemberHandler routes in main.go

Frontend:
- Create member store (fetchMembers by server ID)
- MemberList now fetches members from API instead of empty props
- ChatArea: show channel name instead of ID, add error display,
  add flex-1 + min-w-0 to input for proper sizing, error handling
  on sendMessage

web/src/stores/member.ts

@@ -0,0 +1,37 @@
+import { create } from "zustand";
+import { api } from "../lib/api.ts";
+
+export interface Member {
+  id: string;
+  username: string;
+  display_name: string;
+  avatar: string;
+  status: "online" | "idle" | "dnd" | "offline";
+  status_text: string;
+}
+
+interface MemberState {
+  membersByServer: Record<string, Member[]>;
+  isLoading: boolean;
+  fetchMembers: (serverId: string) => Promise<void>;
+}
+
+export const useMemberStore = create<MemberState>((set) => ({
+  membersByServer: {},
+  isLoading: false,
+
+  fetchMembers: async (serverId) => {
+    set({ isLoading: true });
+    try {
+      const members = await api.get<Member[]>(
+        `/servers/${serverId}/members`,
+      );
+      set((state) => ({
+        membersByServer: { ...state.membersByServer, [serverId]: members },
+        isLoading: false,
+      }));
+    } catch {
+      set({ isLoading: false });
+    }
+  },
+}));