Add live stream connection health tracking and UI TCP timeout warnings

[sappelhoff]

Problem

When stream discovery succeeds over UDP broadcast but establishing the TCP data connection fails or times out (e.g. due to Windows Firewall, multi-NIC/VPN routing like WireGuard, or unreachable remote ports), in->open_stream() caught lsl::timeout_error and silently proceeded to record. The user received no visual feedback in the GUI, resulting in empty ~40 KB files containing only XML stream headers and zero sample chunks.

Solution

1. Stream Telemetry: Added a thread-safe StreamTelemetry tracker in recording to monitor per-stream TCP connection status and sample counts.
2. TCP Timeout Flagging: Flag connection_failed = true when in->open_stream() encounters a timeout error during stream subscription.
3. UI Warning: In MainWindow::statusUpdate(), surface an immediate warning banner in the status bar if any stream's TCP connection failed (or is offline/missing), alerting the experimenter in real-time.

UI Preview

State	Preview
Normal Recording	Recording to sub-001.xdf (00:02:15; 1420kb)
Connection Warning (Timeout / Offline)	Recording to sub-001.xdf (00:02:15; 40kb) | WARNING: Cannot connect to BioSemi (TCP timeout)

Verification

• Tested locally and in CI across Windows, Ubuntu, and macOS.
• Verified that when a stream outlet is dropped/unreachable at start, the UI immediately displays the connection warning banner in the status bar while continuing to record any other valid streams.