gateway: fix dropped audio by sharing one data-session per direction

[Copilot]

In pulse_gateway, audio frames were arriving on each leg's OUTPUT but never made it across to the other leg's INPUT, while video forwarded fine.

Root cause

open_data_sessions() called pulse_data_session_connect_input() twice on the same PULSE_MEDIA_CONTENT_MAIN slot — once with an audio-only config, then once with a video-only config. Each MAIN slot holds exactly one input config (and one output config); the second call silently replaces the first. The input session ended up video-only, so every audio push_frame() was dropped. Same shape of bug on the output side.

Changes in src/gateway.cpp

• Combined configs. Replaced the four per-media config builders with make_input_config() / make_output_config() that describe audio and video together via PULSE_DATA_SESSION_AUDIO_FROM_VALUES_VIDEO_FROM_VALUES (input) and ..._VIDEO_FROM_CAPS (output). open_data_sessions() now issues exactly one connect_input and one connect_output per leg.
• Error logging. PulseError returns from connect_input, connect_output, and push_frame are checked and logged to stderr (push errors are rate-limited 1-in-100). The original silent-drop would have been obvious from day one had these been checked.
• UI string. Fixed the panel blurb claiming the bridge moves "RGBA video and S16LE PCM audio" — it's I420 + F32LE, matching kVideoCaps and the audio config.

The per-frame update_config mechanism for video resolution changes is unchanged: PulseDataSessionFrame carries audio and video PulseDataSessionFrameData side by side, so one shared session per direction is what the API actually expects.