fix: resolve all CI test failures — lazy tkinter, mock network calls

- Lazy-import tkinter in screen_overlay.py (TYPE_CHECKING + runtime
  import) so the module loads on headless Linux CI without libtk8.6
- Fix test_wled_client.py: mock all HTTP endpoints with respx (info,
  cfg, state) instead of hitting real network
- Fix test_calibration.py: assert numpy array shape instead of tuple
- Fix test_processor_manager.py: update to current API (async
  remove_device, dict settings, no update_calibration)
- Fix test_screen_capture.py: get_edge_segments allows more segments
  than pixels

341 tests passing, 0 failures.

server/tests/test_calibration.py

@@ -213,8 +213,8 @@ def test_pixel_mapper_map_border_to_leds():
 
     led_colors = mapper.map_border_to_leds(border_pixels)
 
-    assert len(led_colors) == 40
-    assert all(isinstance(c, tuple) and len(c) == 3 for c in led_colors)
+    assert led_colors.shape == (40, 3)
+    assert led_colors.dtype == np.uint8
 
     # Verify colors are reasonable (allowing for some rounding)
     # Bottom LEDs should be mostly blue