2.0.1

release: v2.0.1 - revert signal client to renderer

Roll back the v2.0.0 main-process signal client. The architecture was
right on paper (lighter unattended footprint when sitting in tray) but
moving the socket out of the renderer broke the offer/answer SDP path
in practice — the renderer-side ConnectionManager still holds the
RTCPeerConnection, and threading every signal:* hop through IPC added
just enough latency that ICE candidates raced ahead of the bridge's
'connect' event on flaky networks. Easier to keep the connection in
the renderer and accept the larger idle footprint.

- Remove main/signal-client.ts and renderer/lib/signal-bridge.ts.
- Restore renderer-owned socket.io client + heartbeat in
  ConnectionManager.
- Restore "minimize to tray" close behaviour (window hidden, not
  destroyed).
- Drop the titanAPI.signal IPC surface from preload.