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/core/lifecycle.py

@@ -0,0 +1,10 @@
+from __future__ import annotations
+
+from app.diagnostics.crash_handler import enable_crash_handler
+from configs.default import LOG_DIR
+
+
+def startup() -> None:
+    enable_crash_handler(LOG_DIR)
+    from app.core.logging import setup_logging
+    setup_logging()