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/shared/src/types.ts

@@ -419,6 +419,12 @@ export interface UpdateFriendRequest {
 
 // ─── Instance Settings Types ────────────────────────────────────────────────
 
+export interface InstanceAdminSettings {
+  instanceName: string;
+  registrationOpen: boolean;
+  discoveryEnabled: boolean;
+}
+
 export interface InstanceStreamingLimits {
   maxBitrateKbps: number;
   minBitrateKbps: number;