Overlay: fix 404, crash on repeat, missing edge test colors, device reset on stop

- Target overlay works without active processing: route pre-loads calibration
  and display info from the CSS store, passes to processor as fallback
- Fix server crash on repeated overlay: replace per-window tk.Tk() with single
  persistent hidden root; each overlay is a Toplevel child dispatched via
  root.after() — eliminates Tcl interpreter crashes on Windows
- Fix edge test colors not lighting up: always call set_test_mode regardless
  of processing state (was guarded by 'not proc.is_running'); pass calibration
  so _send_test_pixels knows which LEDs map to which edges
- Fix device reset on overlay stop: keep idle serial client cached after
  clearing test mode; start_processing() already closes it before connecting

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -11,6 +11,7 @@ from PIL import Image
 
 from wled_controller.api.auth import AuthRequired
 from wled_controller.api.dependencies import (
+    get_color_strip_store,
     get_device_store,
     get_pattern_template_store,
     get_picture_source_store,
@@ -40,7 +41,10 @@ from wled_controller.core.capture.screen_capture import (
     calculate_average_color,
     calculate_dominant_color,
     calculate_median_color,
+    get_available_displays,
 )
+from wled_controller.storage.color_strip_store import ColorStripStore
+from wled_controller.storage.color_strip_source import PictureColorStripSource
 from wled_controller.storage import DeviceStore
 from wled_controller.storage.pattern_template_store import PatternTemplateStore
 from wled_controller.storage.picture_source import ScreenCapturePictureSource, StaticImagePictureSource
@@ -679,6 +683,8 @@ async def start_target_overlay(
     _auth: AuthRequired,
     manager: ProcessorManager = Depends(get_processor_manager),
     target_store: PictureTargetStore = Depends(get_picture_target_store),
+    color_strip_store: ColorStripStore = Depends(get_color_strip_store),
+    picture_source_store: PictureSourceStore = Depends(get_picture_source_store),
 ):
     """Start screen overlay visualization for a target.
 
@@ -694,7 +700,26 @@ async def start_target_overlay(
         if not target:
             raise ValueError(f"Target {target_id} not found")
 
-        await manager.start_overlay(target_id, target.name)
+        # Pre-load calibration and display info from the CSS store so the overlay
+        # can start even when processing is not currently running.
+        calibration = None
+        display_info = None
+        if isinstance(target, WledPictureTarget) and target.color_strip_source_id:
+            try:
+                css = color_strip_store.get_source(target.color_strip_source_id)
+                if isinstance(css, PictureColorStripSource) and css.calibration:
+                    calibration = css.calibration
+                    # Resolve the display this CSS is capturing
+                    from wled_controller.api.routes.color_strip_sources import _resolve_display_index
+                    display_index = _resolve_display_index(css.picture_source_id, picture_source_store)
+                    displays = get_available_displays()
+                    if displays:
+                        display_index = min(display_index, len(displays) - 1)
+                        display_info = displays[display_index]
+            except Exception as e:
+                logger.warning(f"Could not pre-load CSS calibration for overlay on {target_id}: {e}")
+
+        await manager.start_overlay(target_id, target.name, calibration=calibration, display_info=display_info)
         return {"status": "started", "target_id": target_id}
 
     except ValueError as e: