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/diagnostics/crash_handler.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+import faulthandler
+from pathlib import Path
+
+
+def enable_crash_handler(log_dir: str | Path) -> None:
+    log_dir = Path(log_dir)
+    log_dir.mkdir(parents=True, exist_ok=True)
+    crash_log = open(log_dir / "posefit-crash.log", "a", buffering=1)
+    faulthandler.enable(file=crash_log, all_threads=True)