fix(backend): discover user audio session runtime dir for PipeWire support

[thijsdaniels]

Summary

When Decktation runs with "flags": ["root"] under systemd (PluginLoader), the backend process runs as root (UID 0) and does not inherit the desktop session's XDG_RUNTIME_DIR.

Because of this, PortAudio / PipeWire cannot connect to the user session audio daemon at /run/user/<UID>/pipewire-0 and falls back to unpopulated raw ALSA hardware inputs. This caused the audio stream to record silence (leading to VAD filtering out 100% of captured audio and no transcription being generated).

Changes

• Added runtime directory discovery in decktation_backend.py and wow_voice_chat.py before sounddevice / PortAudio is initialized.
• Scans for the active user's runtime directory (/run/user/<UID>/pipewire-0) and sets XDG_RUNTIME_DIR, PIPEWIRE_RUNTIME_DIR, and PULSE_SERVER.
• Allows PortAudio to connect directly to the active PipeWire session as a native client.

Testing

• Tested on Bazzite Linux / SteamOS: verified that PluginLoader connects to PipeWire and captures directly from whatever microphone is selected as default by the desktop/OS (USB headset, standalone mic, etc.), without needing static ALSA hardware overrides.
• Confirmed working both in Decky UI test recording and in-game via controller push-to-talk.