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>

src/presa_mano/api/work_plane_routes.py

@@ -31,7 +31,10 @@ class FitReq(BaseModel):
     v_min: int = Field(..., ge=0)
     u_max: int = Field(..., gt=0)
     v_max: int = Field(..., gt=0)
-    offset_mm: float = Field(5.0, ge=0.0, le=200.0)
+    # offset_mm può essere negativo: valore <0 esclude un buffer SOPRA il
+    # piano (toglie noise depth della superficie); valore >0 è permissivo
+    # (mantieni un buffer sotto il piano). Range esteso simmetricamente.
+    offset_mm: float = Field(5.0, ge=-200.0, le=200.0)
     max_preview_points: int = Field(5000, ge=100, le=20000)