Add railway metrics command with interactive TUI

[rubenszinho]

Closes #242

Adds a new railway metrics command to view service resource usage (CPU, Memory, Network) with both static table and live TUI modes.

New Command

• railway metrics - Display resource metrics for all services.
• --service <name> - Filter by specific service.
• --time <1h|6h|12h|1d|3d|7d> - Historical time range (default: 1h).
• --raw - JSON output for scripting.
• --watch / -w - Live-updating TUI mode.

Features

• Alphabetical service ordering
• Live updates every 5 seconds.
• Keyboard navigation (↑/↓, Tab, q to quit).
• Sparkline charts with units (%, GB, MB).
• Error display in header when API calls fail.
• Color-coded CPU usage and human-readable units.

Development Environment

Updated Nix configuration to support newer Cargo features:

• Added rustup to flake.nix and shell.nix.
• Required for edition2024 feature used by dependencies.
• Fixes compatibility with newer crate versions.

cargo run -- metrics --watch

cargo run -- metrics

cargo run -- metrics --time 6h

cargo run -- metrics --raw