fix(ui): synchronize WebSSH dimensions and normalize action columns

[singchia]

Summary

• Synchronize measured xterm dimensions before SSH open and again on ready/reconnect; avoid resize frames before SSH readiness.
• Fit device/cluster action columns to their content and align headers/actions to the left.
• Share opaque header, row and hover surfaces for sticky action cells across device, Kubernetes and legacy packet tables.
• Document the action-column contract in AGENTS.md and the frontend design language.

Related: #452, #453. Keep both issues open for public verification after labeling resolved.

Validation

• make build-web passed.
• 79 tests passed across XTerminal, DeviceShell, Edges, Clusters and Kubernetes.
• Real SSH through the deployed frontend: initial PTY 147x46, resized 94x29, reconnect 94x29; top rendering verified.
• Deployed device list: dark/light, hover, left alignment and trailing spacing verified.
• Browser fixture validation: Kubernetes Nodes/Workloads/Pods/Events and a low-action cluster list, dark/light, horizontal scrolling and screenshots. This is UI validation, not a real Kubernetes integration test.
• Legacy unrouted list pages were updated and build-checked, not independently browser-tested.

Deployment and rollback

Only nginx on the test environment was updated to ongrid-web:fix-action-tables-20261001. Manager, Edge, protocols and data are unchanged. Restore the prior Compose override/image (ongrid-web:fix-device-actions-20261001-r2) to roll back.

No release tag or website update.

Author confirmation

• I have read and agree to the project contribution guide. Guide: https://github.com/ongridio/ongrid/blob/main/CONTRIBUTING.md