Add auto-initialization to MSS and DXcam engines to fix WGC multi-monitor issue

- MSS and DXcam now auto-initialize on first capture_display() call, matching WGC behavior
- Remove engine.initialize() call from test endpoint to prevent WGC from initializing monitor 0 unnecessarily
- This fixes the issue where testing WGC with secondary monitor would show borders on both displays

Previously, calling engine.initialize() would initialize WGC's monitor 0 by default, then capture_display() would initialize the requested monitor, causing both to be active. Now all engines consistently auto-initialize only when needed.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

server/src/wled_controller/core/capture_engines/dxcam_engine.py

@@ -168,12 +168,12 @@ class DXcamEngine(CaptureEngine):
             ScreenCapture object with image data
 
         Raises:
-            RuntimeError: If not initialized
             ValueError: If display_index doesn't match configured output
             RuntimeError: If capture fails
         """
+        # Auto-initialize if not already initialized
         if not self._initialized:
-            raise RuntimeError("Engine not initialized")
+            self.initialize()
 
         # DXcam is configured for a specific output
         configured_output = self.config.get("output_idx", 0)

server/src/wled_controller/core/capture_engines/mss_engine.py

@@ -115,12 +115,12 @@ class MSSEngine(CaptureEngine):
             ScreenCapture object with image data
 
         Raises:
-            RuntimeError: If not initialized
             ValueError: If display_index is invalid
             RuntimeError: If capture fails
         """
+        # Auto-initialize if not already initialized
         if not self._initialized:
-            raise RuntimeError("Engine not initialized")
+            self.initialize()
 
         try:
             # mss monitors[0] is the combined screen, monitors[1+] are individual displays