Split view, personal modes, budget warnings, server move script

- Up to three chats side by side (#/s/a+b+c), "Open beside" in the sidebar menu
- File manager tabs of a chat stored per user on the server (/sessions/{id}/tabs)
- Personal modes for every user (Settings → My modes, /account/modes)
- Budget meters (total, provider, images, opencode Go windows) with push warnings
  for admins at the warning share and 100 %, shown in the status overlay
- scripts/move-server.sh: move to another Docker host with bind mounts (Unraid);
  scripts/smoke-test.sh

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/main.rs

@@ -72,6 +72,7 @@ async fn main() -> Result<()> {
     update::spawn_checker(app.clone());
     health::spawn_checker(app.clone());
     digest::spawn(app.clone());
+    budget::spawn_alerts(app.clone());
     backup::spawn(app.clone());
     let router = api::router(app);
     let listener = tokio::net::TcpListener::bind(&bind).await?;