feat: instance-level streaming limits with admin settings panel

Add a server-side instance_settings table (single-row, CHECK(id=1))
that stores admin-configurable streaming bounds: bitrate min/max/step,
allowed resolutions, and allowed framerates.

Backend:
- New instance_settings schema + migrations (is_admin on users, default
  settings row, first-registered-user promoted to admin)
- GET/PATCH /api/settings/streaming endpoints with admin-only writes
  and full input validation including cross-field checks

Frontend:
- settingsStore fetches limits on WebSocket ready, exposes isAdmin flag
- ScreenShareSettingsPopover reads bounds from store instead of
  hardcoded constants, auto-clamps stale localStorage values
- buildScreenShareOptions() clamps bitrate to server limits at build
  time as enforcement backstop
- ServerSettings modal gains a "Streaming" tab (admin-only) with
  bitrate range sliders, resolution/framerate toggles, and save/reset

packages/server/src/routes/messages.ts

@@ -22,6 +22,7 @@ function sanitizeUser(row: typeof schema.users.$inferSelect): User {
     avatar: row.avatar,
     status: (row.status ?? 'offline') as User['status'],
     customStatus: row.customStatus,
+    isAdmin: row.isAdmin === 1,
     createdAt: row.createdAt,
   };
 }