Fix 1974: trap focus in Kanban modals

[franksong2702]

Why

Issue #1974 reports that Kanban's task/board modals do not trap keyboard focus, so Tab can move to underlying page controls while the modal is open.

What changed

• Added a shared _trapModalFocus(modalEl) helper in static/panels.js.
• Wired _trapModalFocus into:
  • openKanbanCreate()
  • openKanbanEdit()
  • openKanbanCreateBoard()
  • openKanbanRenameBoard()
• Added per-modal cleanup handles so listeners are removed on close/reopen.
  • _kanbanTaskModalFocusCleanup
  • _kanbanBoardModalFocusCleanup
• Extended static regression coverage in tests/test_kanban_ui_static.py:
  • helper existence
  • modal install/remove assertions for task modal open/close
  • modal install/remove assertions for board modal open/close

Verification

• node --check static/panels.js
• node --check static/ui.js
• pytest -q tests/test_kanban_ui_static.py (39 passed)
• pytest -q tests/test_kanban_ui_static.py tests/test_kanban_bridge.py::test_patch_status_running_is_rejected_to_protect_dispatcher_contract tests/test_kanban_bridge.py::test_patch_status_done_to_running_is_rejected (39 passed)

Risks

• Focusability relies on querySelectorAll + computed styles; edge cases with dynamically injected / hidden controls should still default to the current focus trap behavior.
• Scope is intentionally limited to focus trapping and listener cleanup, leaving broader modal/refactor work to follow-up issues.