feat: add Node tab for viewing node information

[sarub0b0]

Summary

Add a new top-level "Node" tab to kubetui for viewing Kubernetes Node information.

Motivation

Users managing NVIDIA GPU nodes need to frequently check node labels (e.g., nvidia.com/mig.config.state) and status. Currently this requires kubectl --show-labels and external tools like gron.

Features

List Table

• Default columns: Name, Status, Roles, Age, Version (kubectl get nodes equivalent)
• Customizable columns via existing column configuration

Detail Pane

• Node YAML display (kubectl get node -o yaml equivalent)
• Related Pods list below YAML (kubectl get pods --field-selector spec.nodeName=<node> equivalent)
• Scrollable text with existing search functionality

Filter (follows Pod container log filter pattern)

• node:<regex> — filter by node name
• !node:<regex> — exclude by node name
• label:key=value — label selector filter
• Filter mode: show only matching nodes (default)
• Highlight mode: show all nodes, highlight matches

Design Document

See docs/plans/2026-02-09-node-tab-design.md