fix: branching forms with visibilityCriteria render the hidden next step

[christianmat]

Summary

• Fixes a bug where branching forms with visibilityCriteria depending on the just-submitted step's form data would still render the now-hidden next step instead of skipping past it. Reported by a customer on flow_dVcRW58i (docs example for branching forms).
• Root cause: in internalComplete (packages/js-api/src/core/flow.ts), after the COMPLETED_STEP/SKIPPED_STEP response refreshed state, the client fired a follow-up STARTED_STEP for the optimistic next step (chosen from pre-request visibility). When the server hid that step via visibilityCriteria after seeing the form data, the spurious STARTED_STEP flipped currentStepId back to the now-hidden step. Verified by replaying the API sequence against prod with the customer's flow.
• Fix: after the refresh, derive the next-step event from this.getCurrentStep() rather than the pre-request nextStepForStartEvent. The non-branching behavior added in 2723c81 is preserved (the new regression test guards it).
• Changeset bumps both @frigade/js and @frigade/react as patches so customers pick up the fix on a normal yarn upgrade @frigade/react.

Test plan

• New failing test in packages/js-api/test/flow-branching-visibility.test.ts reproduces the bug, then passes after the fix.
• New regression test verifies non-branching flows still send STARTED_STEP for the next step (preserves 2723c81).
• yarn test in packages/js-api: 28 passed, 0 failed (existing flow-flicker and integration tests unaffected).
• Manually verify in Storybook against the customer's flow that picking "Go to page 3" lands on page 3 (already verified locally pre-PR).

🤖 Generated with Claude Code