Show captured border width overlay in picture CSS test preview

Backend: send border_width in WS metadata and frame_dims (width, height)
as a separate JSON message on first JPEG frame.

Frontend: render semi-transparent green overlay rectangles on each active
edge showing the sampling region depth, plus a small px label. Overlays
are proportionally sized based on border_width relative to frame dimensions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/api/routes/color_strip_sources.py

@@ -822,6 +822,7 @@ async def test_color_strip_ws(
                     indices = [(idx + offset) % total for idx in indices]
                 edges.append({"edge": seg.edge, "indices": indices})
             meta["edges"] = edges
+            meta["border_width"] = cal.border_width
         if is_composite and hasattr(source, "layers"):
             # Send layer info for composite preview
             enabled_layers = [l for l in source.layers if l.get("enabled", True)]
@@ -850,6 +851,7 @@ async def test_color_strip_ws(
             _frame_live = stream.live_stream
         _last_aux_time = 0.0
         _AUX_INTERVAL = 0.08  # send JPEG preview / brightness updates ~12 FPS
+        _frame_dims_sent = False  # send frame dimensions once with first JPEG
 
         # Stream binary RGB frames at ~20 Hz
         while True:
@@ -904,8 +906,16 @@ async def test_color_strip_ws(
                             # Ensure 3-channel RGB (some engines may produce BGRA)
                             if img.ndim == 3 and img.shape[2] == 4:
                                 img = img[:, :, :3]
-                            # Downscale for bandwidth
                             h, w = img.shape[:2]
+                            # Send frame dimensions once so client can compute border overlay
+                            if not _frame_dims_sent:
+                                _frame_dims_sent = True
+                                await websocket.send_text(_json.dumps({
+                                    "type": "frame_dims",
+                                    "width": w,
+                                    "height": h,
+                                }))
+                            # Downscale for bandwidth
                             scale = min(960 / w, 540 / h, 1.0)
                             if scale < 1.0:
                                 new_w = max(1, int(w * scale))