feat(desktop-main): SDK bootstrap of DynamoDB lock table

[CoderCoco]

Child of #139 (epic: first-run wizard + credentials UX). Authoritative scope in the design spec.

Scope

Programmatic creation of the terraform state lock table via @aws-sdk/client-dynamodb. CreateTable with LockID primary key. Idempotent.

Acceptance

• Lock table exists after the wizard step.
• Table is usable as the terraform state lock.