fix: disable Rocket signal handling to prevent zombie process

[lohrm-spf]

Summary

• Disables Rocket's built-in ctrlc and Unix signals shutdown triggers in the asset server configuration.
• Fixes the single-instance handler to recreate widgets when re-launched after all windows were destroyed externally.

Problem

Zombie path 1: Rocket signal handling

Rocket 0.5 registers signal handlers (SIGTERM by default on Unix, SIGINT via ctrlc) that replace the OS defaults. When an external process manager (e.g. GlazeWM) sends SIGTERM to Zebar:

1. Rocket catches SIGTERM and gracefully shuts down the HTTP asset server
2. launch().await returns Ok(()) — no error is logged
3. Tauri's event loop + placeholder window keep the process alive
4. Zebar is now a zombie: running but without an asset server, so widgets cannot render
5. The tauri-plugin-single-instance lock is still held by the zombie
6. Any new zebar instance detects the lock, communicates with the zombie, and exits
7. Result: no working Zebar, no way to start a new one without manually killing the zombie

Zombie path 2: External window destruction

When a window manager restarts (e.g. GlazeWM during development), it can destroy Zebar's widget windows externally. The placeholder window keeps the process alive, but widget_states is now empty. If the WM then re-launches zebar:

1. New instance detects the single-instance lock
2. CLI args forwarded to the existing instance as CliCommand::Empty
3. The single-instance handler unconditionally no-ops for Empty
4. Result: process alive with asset server running, but zero visible widgets and no way to recover without manually killing the process

Fix

Rocket signal handling

Disable Rocket's signal handling so SIGTERM/SIGINT fall through to the OS default (process termination). The asset server lifecycle is now tied to the process — when the process dies, everything dies, and the single-instance lock is released.

• shutdown.ctrlc = false — Rocket won't intercept Ctrl-C
• shutdown.signals = [] (Unix only) — Rocket won't intercept SIGTERM

Normal Tauri-initiated shutdown still works: when the Tauri event loop exits, the tokio runtime is dropped, cancelling the Rocket background task.

Single-instance widget recovery

The Empty command now only no-ops when widgets are still alive. If widget_states is empty, startup() is called to recreate widgets from the startup config.