fix(webssh): frame terminal workspace and add fullscreen controls

[singchia]

Summary

• Add responsive workspace gutters, a separate toolbar and a subtly framed terminal with internal spacing.
• Keep viewport-responsive terminal height and existing PTY size synchronization.
• Add native fullscreen controls without recreating SSH sessions; allow exiting fullscreen even after disconnection.
• Exit fullscreen before opening the portal-based reconnect dialog and display fullscreen errors without closing SSH.

Related: #455. Keep the issue open for public verification after marking resolved.

Validation

• 12 focused XTerminal/DeviceShell tests passed, including fullscreen denial without SSH teardown.
• make build-web passed for the accepted trial layout; CI builds the final patch.
• Real SSH through deployed trial frontend: top, resize, fullscreen entry/exit and reconnect passed. Initial 139x43, resized/reconnected 86x26.
• Dark/light screenshots inspected; maintainer accepted the deployed layout.
• No backend, protocol or Edge changes.

Rollback

Restore the previous frontend image ongrid-web:fix-action-tables-20261001. No data migration.

Author confirmation

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