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>

web/work_plane.js

@@ -246,6 +246,21 @@ function renderDetails(plane) {
   $('wpDetails').textContent = JSON.stringify(plane, null, 2);
 }
 
+// FastAPI 422 returns `detail` as an array of validation errors:
+//   [{loc, msg, type, ...}, ...]
+// Plain HTTPException returns it as a string. Render either case usefully so
+// the status chip never says "[object Object]".
+function formatDetail(detail) {
+  if (typeof detail === 'string') return detail;
+  if (Array.isArray(detail)) {
+    return detail.map(e => {
+      const loc = Array.isArray(e.loc) ? e.loc.join('.') : e.loc;
+      return loc ? `${loc}: ${e.msg}` : (e.msg || JSON.stringify(e));
+    }).join('; ');
+  }
+  return JSON.stringify(detail);
+}
+
 async function doFit() {
   if (!roi) return;
   setChip('fit…');
@@ -259,7 +274,7 @@ async function doFit() {
       method: 'POST', headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify(body),
     }).then((r) => r.json());
-    if (r.detail) throw new Error(r.detail);
+    if (r.detail) throw new Error(formatDetail(r.detail));
     previewPlane = r.plane;
     previewPoints = r.points_camera;
     three.setPlane(previewPlane);
@@ -282,7 +297,7 @@ async function doSave() {
       method: 'POST', headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ plane: previewPlane }),
     }).then((r) => r.json());
-    if (r.detail) throw new Error(r.detail);
+    if (r.detail) throw new Error(formatDetail(r.detail));
     savedPlane = r.plane;
     previewPlane = null;
     $('wpStateBlock').innerHTML = `piano salvato · RMS ${savedPlane.fit_rms_mm.toFixed(1)} mm · n_inliers ${savedPlane.n_inliers} · offset ${savedPlane.offset_mm} mm`;