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/main.py

@@ -1,8 +1,8 @@
 """A dora webcam source: captures frames and publishes them as std/media/v1/Image.
 
 Each `tick` reads one frame, optionally flips and resizes it, encodes it, and
-sends it. The node stops when the tick input closes, or after CAPTURE_FRAMES
-frames if that is set.
+sends it. The node stops when the tick input closes, after CAPTURE_FRAMES
+frames if that is set, or when `q` is pressed in the preview window.
 """
 
 import argparse
@@ -11,7 +11,7 @@ import os
 import cv2
 from dora import Node
 
-from dora_webcam import camera, encode, image
+from dora_webcam import camera, encode, image, preview
 
 FLIPS = {
     "VERTICAL": 0,
@@ -43,6 +43,8 @@ def main() -> None:
     # 0 runs until the tick closes; a positive count is what lets CI run the
     # dataflow to completion instead of killing it on a timeout.
     limit = _int("CAPTURE_FRAMES") or 0
+    # Never under CI: there is no display there, and imshow would fail.
+    previewing = os.getenv("PREVIEW", "").lower() in ("1", "true") and not ci
 
     node = Node(args.name)
     sent = 0
@@ -66,6 +68,11 @@ def main() -> None:
             if width and height and (frame.shape[1], frame.shape[0]) != (width, height):
                 frame = cv2.resize(frame, (width, height))
 
+            # After the flip and the resize, so what is on screen is what is
+            # published rather than what came off the sensor.
+            if previewing and preview.show(frame):
+                break
+
             # Taken before encoding: a compressed payload's byte count is not
             # its pixel dimensions.
             frame_height, frame_width = frame.shape[:2]
@@ -87,6 +94,9 @@ def main() -> None:
     finally:
         capture.release()
 
+        if previewing:
+            preview.close()
+
 
 def _parse_args() -> argparse.Namespace:
     parser = argparse.ArgumentParser(