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/server/src/db/migrate.ts

@@ -69,7 +69,8 @@ export function runMigrations(db: Database.Database): void {
       columns: [
         { name: 'instance_name', type: "TEXT DEFAULT 'Backspace'" },
         { name: 'worker_id', type: 'INTEGER' },
-        { name: 'discovery_enabled', type: 'INTEGER NOT NULL DEFAULT 1' }
+        { name: 'discovery_enabled', type: 'INTEGER NOT NULL DEFAULT 1' },
+        { name: 'registration_open', type: 'INTEGER' }
       ]
     },
     {