feat: dedicated Instance Settings modal with admin controls

Move instance-level administration out of Space Settings into its own
modal. Adds admin UI for instance name, registration toggle, and
discovery toggle. Streaming limits panel relocated from SpaceSettings.

- Add InstanceAdminSettings type and GET/PATCH /api/settings/instance
- Add registration_open DB column (nullable, env var fallback)
- Auth registration and instance info now check DB override
- New InstanceSettings modal with General and Streaming tabs
- Admin shield button in UserAreaPanel (visible to admins only)
- Remove Streaming tab from SpaceSettings

packages/web/src/components/layout/AppLayout.tsx

@@ -12,6 +12,7 @@ import { CreateChannelModal } from '../modals/CreateChannel';
 import { InviteModal } from '../modals/InviteModal';
 import { UserSettingsModal } from '../modals/UserSettings';
 import { SpaceSettingsModal } from '../modals/SpaceSettings';
+import { InstanceSettingsModal } from '../modals/InstanceSettings';
 import { ChannelSettingsModal } from '../modals/ChannelSettingsModal';
 import { NewDmModal } from '../modals/NewDmModal';
 import { AddDmMemberModal } from '../modals/AddDmMemberModal';
@@ -273,6 +274,7 @@ export function AppLayout() {
       <InviteModal />
       <UserSettingsModal />
       <SpaceSettingsModal />
+      <InstanceSettingsModal />
       <ChannelSettingsModal />
       <NewDmModal />
       <AddDmMemberModal />