feat(ai): add action-plan preview and explicit execution confirmation #90

Turn a validated local model plan into a reviewable OffPDF operation.

Scope

• Show selected files, page ranges, options, and output names before execution.
• Let users adjust supported values or cancel the plan.
• Require a separate confirmation before invoking the backend.
• Use the existing job progress, error, and cancellation flows.
• Never overwrite the source file.

Acceptance criteria

• Generating a plan performs no PDF mutation.
• Only the confirmed, validated values reach a backend command.
• Unsupported requests remain editable text and show a clear explanation.
• Preview and confirmation states have component tests.