feat(tui): navigate panels via Up/Down arrow overflow at list boundaries

[varshaprasad96]

Summary

When the cursor reaches the boundary of a TUI dashboard panel list, Up/Down arrow keys now overflow into the adjacent panel instead of clamping silently. Down at the bottom of a panel moves focus to the next panel (cursor at first item); Up at the top moves to the previous panel (cursor at last item). Empty panels are skipped. The ring wraps: Gateways → Providers/GlobalSettings → Sandboxes → Gateways.

Related Issue

Closes #2273

Changes

• Added DASHBOARD_PANELS const array defining the panel ring order
• Added panel_item_count() helper that returns item count for any panel, respecting the active middle-pane tab (Providers vs GlobalSettings)
• Added set_panel_cursor() helper to set the selected index for any panel
• Added overflow_focus_down() and overflow_focus_up() methods that walk the panel ring to find the next/previous non-empty panel
• Modified all four dashboard key handlers (handle_gateways_key, handle_providers_key, handle_global_settings_key, handle_sandboxes_key) to call overflow helpers at list boundaries instead of clamping

Testing

• rustfmt --edition 2024 --check passes
• cargo check -p openshell-tui compiles cleanly
• Unit tests added/updated — App::new requires a gRPC client, making isolated unit tests impractical without test infrastructure changes
• E2E tests added/updated (if applicable)
• Manually tested with a live gateway and sandbox:
  • Down at bottom of each panel → moves to next panel (cursor at first item)
  • Up at top of each panel → moves to previous panel (cursor at last item)
  • Wrap-around works in both directions
  • Empty panels are skipped
  • Mid-list Up/Down scrolls normally within a panel
  • TAB navigation unaffected
  • Middle pane tab switching (h/l) unaffected
  • GlobalSettings tab respects overflow correctly

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[github-actions]

Thank you for your submission! We ask that you sign our Developer Certificate of Origin before we can accept your contribution. You can sign the DCO by adding a comment below using this text:

---

I have read the DCO document and I hereby sign the DCO.

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the DCO Assistant Lite bot.}

[varshaprasad96]

I have read the DCO document and I hereby sign the DCO.