build with uv, and preview locally instead of a second dataflow

The build command was `pip install .`, which is what upstream and the dora docs
use because `--uv` rewrites it. Spelling it `uv pip install .` means the
dataflow never installs through pip at all, whether or not `--uv` was passed.
The cost is that the node then needs a virtualenv to install into, so it must
be run with `--uv`; the README says so.

The second dataflow is gone. It existed to wire this node into dora-image-view
just to see the frames, which pulled another repository over the network and
needed a tag there. PREVIEW=true now opens a local window of the frames as they
are published, so checking a camera, an encoding or a flip needs this
repository alone. It shows the frame after the flip and the resize, so what is
on screen is what is sent, and it never opens under CI, where there is no
display.

That a source node can display its own output overlaps with what a viewer node
does. The overlap is the point: the alternative was a second repository in the
loop before you can tell whether a camera works.

dora_webcam/preview.py

@@ -0,0 +1,36 @@
+"""An optional local preview window.
+
+A source node showing its own frames overlaps with what a viewer node does, and
+that is the point: it means checking that a camera works needs this repository
+only, not a second node wired into a dataflow. It is off unless asked for, and
+never opens under CI.
+"""
+
+import cv2
+import numpy as np
+
+WINDOW = "dora-webcam"
+
+
+def show(frame: np.ndarray) -> bool:
+    """Display one frame and report whether the viewer asked to quit.
+
+    Parameters
+    ----------
+    frame : np.ndarray
+        The captured BGR frame, before encoding, which is what a person wants
+        to look at.
+
+    Returns
+    -------
+    bool
+        True when `q` was pressed, which stops the node.
+
+    """
+    cv2.imshow(WINDOW, frame)
+    return cv2.waitKey(1) & 0xFF == ord("q")
+
+
+def close() -> None:
+    """Tear the window down, if one was ever opened."""
+    cv2.destroyAllWindows()