feat: add AReno dashboard

[xsuler]

Summary

Add a first-party AReno dashboard for job management, runtime observability, metrics inspection, logs, rollout samples, and agent-assisted operations.

Details

• Adds an areno dashboard command surface for starting and stopping the dashboard.
• Adds a backend-served React dashboard with job list and job detail views.
• Registers AReno train and serve jobs so externally launched tasks can be observed without stdout redirection.
• Shows per-job config, runtime state, logs, rollout samples, TensorBoard metric series, and time-performance views.
• Adds an agent panel with tools for listing jobs, inspecting jobs, fetching metrics, checking runtime state, and starting/stopping tasks.
• Supports relative backend-served assets for reverse-proxy path prefixes.

Closes #160

[gemini-code-assist]

Code Review

This pull request introduces a React-based dashboard for AReno, enabling users to monitor training and serving tasks, inspect metrics, view rollout samples, and interact with an operations agent. It adds a background dashboard server, a lightweight process registry, and integrates state-tracking hooks across SFT, DPO, PPO, and policy-only trainers. The review feedback highlights several critical robustness and security improvements, including avoiding permission conflicts with hardcoded /tmp paths, handling potential subprocess errors when querying the environment, replacing unsafe preexec_fn usage with start_new_session=True, optimizing file reading to prevent OOM crashes on large files, and securing ripgrep searches against option injection.

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