feat(frontend): add confirm button to node config modal

[LukasHirt]

Summary

• The node configuration modal (NodeDetailsPanel.vue) previously closed only via clicking the overlay backdrop, the header X icon, or pressing Escape — none of which are very discoverable as a "confirm and finish configuring" action.
• Adds an explicit Done button in a footer area, using oc-button (variation="primary") for visual consistency with the rest of the panel and the app's existing conventions (e.g. the Save button in WorkflowBuilder.vue).
• Since the panel's fields already live-bind to node.data via the existing field()/actionParam() helpers and emit update immediately, the new button's only job is to emit the same close event the existing close affordances already emit, which WorkflowBuilder.vue handles by nulling out selectedNodeId.

Test plan

• Added frontend/tests/unit/NodeDetailsPanel.spec.ts (new — first component-mount test in this repo, establishes the @vue/test-utils + vue3-gettext mounting pattern for future component tests):
  • Confirms the Done button is rendered.
  • Confirms clicking it emits close exactly once.
• Verified test fails without the change (button doesn't exist) and passes after implementing it.
• cd frontend && npm run test:unit — all 3 test files / 6 tests pass.
• cd frontend && npm run check:types — clean.
• cd frontend && npm run lint — clean.

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