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/schema.ts

@@ -192,6 +192,7 @@ export const instanceSettings = sqliteTable('instance_settings', {
   allowedFramerates: text('allowed_framerates').notNull().default('30,45,60'),
   maxResolution: integer('max_resolution').notNull().default(1080),
   maxFramerate: integer('max_framerate').notNull().default(60),
+  registrationOpen: integer('registration_open'),  // null = use env var default, 0/1 = explicit
   updatedAt: integer('updated_at').notNull(),
 });