feat: ship default calibration (2440x1220 bed homography)

Persist a working calibration so the overlay is usable out-of-the-box without
each viewer re-calibrating. 4 corner point-pairs + precomputed homography
(machine-mm -> normalized [0,1] image coords).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

public/config.json

@@ -1,5 +1,23 @@
 {
   "streamUrl": "/camera/mjpg/video.mjpg",
-  "calibration": null,
+  "calibration": {
+    "imagePoints": [
+      [0.3182042321463028, 0.008781558726673985],
+      [0.6584910919108977, 0.010976948408342482],
+      [0.9349741654696311, 0.9341383095499451],
+      [0.05201209184657934, 0.9407244785949506]
+    ],
+    "machinePoints": [
+      [0, 0],
+      [0, 1220],
+      [2440, 1220],
+      [2440, 0]
+    ],
+    "homography": [
+      -0.00012223053029634107, 0.00028168511954120923, 0.3182042321463022,
+      0.00014436860873395254, 0.0000018455329325253353, 0.008781558726673699,
+      -0.0002525449441254382, 0.000004193623923514485, 1
+    ]
+  },
   "renderDefaults": { "cutColor": "#00ffff", "rapidColor": "#ff7700", "lineWidth": 3 }
 }