fix: initialize action argument values from defaults in node modal - #257

Summary

• Fixes checkboxes on the node modal not reflecting the default values of action arguments (Bug: check boxes on node modal on dashboard don’t match default values #190)
• When action arguments are rendered, item.value is now initialized from item.default if the value hasn't been set yet
• Removes redundant :value prop from v-checkbox (unnecessary with v-model + :true-value/:false-value)

Details

The ArgumentDefinition objects from the backend include a default field, but the frontend never used it to initialize the editable value field. This caused boolean checkboxes to always appear unchecked regardless of the action's declared default. The fix also benefits text and number fields by pre-filling them with their defaults.

Test plan

• Open the dashboard and navigate to a node modal with boolean action arguments
• Verify checkboxes reflect the correct default values (checked if default: true, unchecked if default: false)
• Verify text and number fields also show their default values pre-filled
• Verify that manually changing a value still works correctly
• Verify "Send Action" and "Show Copyable Workflow Step" still use the correct values

Closes #190

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