feat: UI for server/channel creation, settings link, role management backend

Frontend:
- Settings link in header next to logout
- [+] button to create server (name + optional icon)
- [#] button to join server via invite code
- [+] button to create channel (name, type, category)
- Server name shown instead of UUID in channel list header
- /invites/:code route wired for JoinServer component

Backend:
- Wire role CRUD + member-role assignment routes (MANAGE_SERVER gated)
- Seed @everyone default role on server creation

web/src/components/Layout.tsx

@@ -1,5 +1,5 @@
 import { useEffect } from 'react';
-import { Outlet, useLocation, useNavigate } from 'react-router-dom';
+import { Link, Outlet, useLocation, useNavigate } from 'react-router-dom';
 import { useAuthStore } from '../stores/auth.ts';
 import { ServerBar } from './ServerBar.tsx';
 import { ChannelList } from './ChannelList.tsx';
@@ -46,6 +46,7 @@ export function Layout() {
             <span>
               USER: <span className="text-gb-aqua">{user?.username || 'unknown'}</span>
             </span>
+            <Link to="/settings" className="terminal-button text-xs">[SETTINGS]</Link>
             <button onClick={() => logout().then(() => navigate('/login'))} className="terminal-button text-xs">
               [LOGOUT]
             </button>