fix(ui): What-If Planner submits empty assumption dates → backend 422

[w7-mgfcode]

Summary

On /visualize/planner, enabling a Price change or Promotion assumption but leaving its From / To date inputs blank submits empty strings, and the backend rejects the request with an RFC 7807 422 (4 validation errors). The error is shown gracefully in red below Run simulation (no crash), but a user has no in-form signal that the dates are required.

Steps to reproduce

1. Open /visualize/planner, let a baseline predict job auto-select.
2. Tick Price change (and/or Promotion) — leave the From/To date fields empty.
3. Click Run simulation.
4. Backend returns 422 — Request validation failed with 4 error(s).

Root cause

buildAssumptions() in frontend/src/pages/visualize/planner.tsx always sends start_date / end_date from the form state, which initialise to '' (useState('') at lines ~82-88). Empty strings fail the Pydantic date validation on PriceAssumption / PromotionAssumption.

Suggested fix

Either:

• Default priceStart/priceEnd/promoStart/promoEnd to the forecast window (baseline train_end + 1 … + horizon) once a baseline job is selected; or
• Disable Run simulation / show inline field validation until an enabled assumption has both dates filled.

Found by

Real-browser dogfood of PRP-27 (/visualize/planner) on dev after PR #227.

[w7-mgfcode]

Verified shipped on dev. Fix commit: 34104c9 fix(ui): block planner runs with empty assumption dates (#228). Code confirms: planner.tsx exposes dateErrors.priceStart / priceEnd / promoStart / promoEnd inline validation (lines 340-352) so empty-date submission is now blocked at the form layer before the backend would 422.