feat(monitor): attach dashboards to a background proxy

[srnnkls]

Summary

Running the proxy as a service currently loses the interactive monitor: the TUI owns the listener, while plain mode does not collect monitor history. This adds claude-code-proxy monitor to attach a dashboard to an existing proxy. Closing that dashboard leaves inference and other dashboards running.

• Keep monitor accounting in plain serve --no-monitor mode and handle Ctrl-C/SIGTERM with graceful service shutdown.
• Expose versioned, read-only snapshots at GET /monitor, restricted to actual loopback peers. Immutable read models carry elapsed durations and computed rates without reconstructing process-local clocks.
• Poll outside the terminal event loop, retain the last snapshot during outages, and reconnect automatically. q and Ctrl-C detach; dropping a dashboard cancels its polling task.

# Run under a service manager or in another terminal.
claude-code-proxy serve --no-monitor

# Attach one or more dashboards.
claude-code-proxy monitor
claude-code-proxy monitor --url http://127.0.0.1:19999

The existing integrated serve dashboard keeps its shutdown behavior. History remains in memory and resets on service restart. Remote hosts can be reached through an SSH port forward. The attached dashboard needs no provider credentials and has no server mutation or shutdown endpoint.

This branch is based directly on upstream main and is independent of #133.

Validation

• cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo build --all-targets.
• cargo test --all-targets, including snapshot serialization, loopback access checks, multiple viewers, detach/reattach, polling failure/recovery, cancellation, and incompatible protocol handling.
• Real pseudo-terminal smoke test: start the headless proxy, attach and quit with q, reattach and quit with Ctrl-C, verify HTTP health after each detach, then verify clean SIGTERM shutdown.

Also ran the edition and Clippy fixers. Validation uses local mock servers; it makes no live provider requests.