fix: update multistep labels reactively

[justin-schroeder]

What changed

• Updates multi-step stepName whenever a child step’s label prop changes, instead of copying the label only once when the step is registered.
• Adds a regression that changes step labels after mount and verifies the tab label text updates, covering i18n/reactive label use cases.

Verification

• pnpm exec vitest run --config /tmp/formkit-vitest-ci.mts packages/addons/__tests__/multistep.spec.ts -t "#1377" --reporter verbose
• pnpm exec vitest run --config /tmp/formkit-vitest-ci.mts packages/addons/__tests__/multistep.spec.ts --reporter verbose
• pnpm vitest --typecheck.only --run
• node scripts/cli.mjs --script build addons reached successful CJS/ESM addon bundles, then failed during DTS on the existing @formkit/auto-animate package exports/type resolution issue in autoAnimatePlugin.ts.

Security

• No new inputs, storage, network, or data exposure paths. This only keeps existing display text in sync with reactive label props.

Closes #1377