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/signaling/ice_parser.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+import re
+from typing import Any
+
+from aiortc import RTCIceCandidate
+
+
+def parse_ice(data: dict[str, Any]) -> RTCIceCandidate | None:
+    match = re.match(
+        r'candidate:(\S+) (\d) (\S+) (\d+) (\S+) (\d+) typ (\S+)(?: raddr (\S+) rport (\d+))?',
+        data["candidate"],
+    )
+    if not match:
+        return None
+    g = match.groups()
+    cand = RTCIceCandidate(
+        foundation=g[0],
+        component=int(g[1]),
+        protocol=g[2].lower(),
+        priority=int(g[3]),
+        ip=g[4],
+        port=int(g[5]),
+        type=g[6],
+        relatedAddress=g[7],
+        relatedPort=int(g[8]) if g[8] else None,
+    )
+    cand.sdpMid = data.get("sdpMid")
+    cand.sdpMLineIndex = data.get("sdpMLineIndex", 0)
+    return cand