Feat/ssh guide modal root reset

[hololee]

PR Title

feat(dashboard/settings): improve SSH guide + root password reset flow and add safe host-SSH reset

Summary

This PR refines the Dashboard SSH guide and root password reset flow, adds a safe reset action for Host
Server Connection in Settings (with confirmation modal), and ensures JupyterLab terminals prefer zsh.

What Changed

1) Dashboard SSH guide improvements

• SSH guide now reflects configured SSH client values (host/port/user).
• Hides redundant guide notes when host SSH is already configured.
• Improved SSH guide modal UX and copy behavior.

2) Root password reset flow hardening

• Improved Dashboard input UX for root password reset.
• Normalized worker routing for root password reset so host/worker behavior is consistent.
• Improved error mapping and handling for clearer failure behavior.
• Hardened exec IO handling used during reset operations.

3) Settings: Host Server Connection reset

• Added Reset action in Host Server Connection.
• Added confirmation modal before reset.
• Reset scope includes:
  • Stored SSH client config
  • Browser-stored encrypted key and key metadata
  • SSH form state
  • Related terminal/session UI state
• Reset action style aligned with destructive action buttons.

4) JupyterLab terminal shell fallback

• Added --ServerApp.terminado_settings in runtime command.
• Shell priority: zsh -> bash -> sh.

Scope

Frontend

• frontend/src/pages/Dashboard.tsx
• frontend/src/pages/Settings.tsx
• frontend/src/utils/sshGuide.ts
• i18n updates (en, ko)

Backend

• backend/app/routers/environments.py
• backend/app/routers/worker_api.py
• backend/app/tasks.py
• related tests

Validation

• Pre-commit checks passed:
  • Frontend lint/build
  • Backend lint/tests for affected scope
• Manual checks:
  • SSH guide modal rendering/copy/config reflection
  • Root password reset on host and worker paths
  • Host Server Connection reset confirmation + data clearing
  • JupyterLab terminal shell preference (zsh first)

Checklist

• Functional behavior verified
• i18n (en/ko) updated
• Key regression paths reviewed
• Lint/tests passed