Add inline validation for action editor configuration issues

[Copilot]

The action editor did not surface configuration problems until an action was executed. This change adds deterministic, per-action validation so editing immediately shows blocking errors and non-blocking warnings without triggering side effects.

• Validation model

  • Adds ActionValidator and structured validation issues with Error vs Warning severity.
  • Covers unsupported action types, missing action selection, invalid custom shortcuts, missing shell targets, and invalid working directories.
  • Treats unresolved shell targets as warnings so shell-resolvable targets remain possible.

• Editor feedback

  • Updates ActionEditorViewModel to recompute validation whenever the selected action or its editable fields change.
  • Surfaces issues inline in ActionEditorView, scoped to the selected action rather than the action list as a whole.
  • Uses distinct visual treatment for errors and warnings to make run-blocking problems easy to spot.

• Behavioral coverage

  • Adds deterministic tests for validator behavior across key, shell, and unsupported action cases.
  • Adds view-model tests to cover live validation updates while editing.

var issues = ActionValidator.Validate(action);

if (issues.Any(issue => issue.Severity == ActionValidationSeverity.Error))
{
    // Blocked until configuration is corrected.
}

[danielchalmers]

Very poorly styled. Would probably look better as helper text under the textboxes themselves

We already have tray notifications for bad actions so that'll work fine for now.