fix(work_plane): allow negative offset_mm + readable validation errors

- Drop the ge=0 constraint on FitReq.offset_mm (negative values are a
  documented usage: aggressive noise cull above the plane).
- HTML input min lowered to -200 to match.
- Render FastAPI 422 validation arrays as a human-readable string in the
  status chip (was showing "[object Object]").

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/index.html

@@ -205,7 +205,7 @@
         </div>
         <div class="kv compact">
           <label>ROI <span id="wpRoiInfo" class="dim">— · clicca 2 angoli sull'RGB</span></label>
-          <label>offset (mm) <input type="number" id="wpOffset" value="5.0" step="0.5" min="0" max="200" /></label>
+          <label>offset (mm) <input type="number" id="wpOffset" value="5.0" step="0.5" min="-200" max="200" /></label>
         </div>
         <div class="actions">
           <button id="wpFitBtn" class="primary" disabled>Calcola piano (preview)</button>