Refactor into modular app structure

Split monolithic files into focused modules:
- app/core: settings, logging, lifecycle
- app/signaling: websocket server, ICE parser, message models
- app/webrtc: peer session, video receiver, frame source
- app/vision: pose landmarker wrapper, model config, pose types
- app/exercises/dead_bug: detector, metrics, rules, state machine, types
- app/rendering: skeleton renderer, status overlay, window display
- app/audio: rep announcer
- app/diagnostics: perf timer, crash handler
- configs: environment-based settings
- tests: unit tests for rules, state machine, ICE parser
- run.py: entry point

app/rendering/window_display.py

@@ -0,0 +1,21 @@
+from __future__ import annotations
+
+import cv2
+
+WINDOW_NAME = "Android Camera (WebRTC)"
+
+
+def show_frame(image, window_name: str = WINDOW_NAME) -> None:
+    cv2.imshow(window_name, image)
+
+
+def wait_key(delay_ms: int = 1) -> int:
+    return cv2.waitKey(delay_ms) & 0xFF
+
+
+def is_esc_pressed() -> bool:
+    return wait_key(1) == 27
+
+
+def close_window() -> None:
+    cv2.destroyAllWindows()