feat: capture desktop apps with PocketStation

[Raphjacksun7]

Summary

Adds PocketStation as an optional source for one desktop application's audio. MeetGraph keeps its current microphone capture, speech segmentation, transcription, diarization, storage, and knowledge-graph code.

Install and select it in the existing Audio sources panel:

pip install 'meetgraph[pocketstation]'
meetgraph

Enable Desktop application (PocketStation), enter an application name such as Zoom, and keep Microphone enabled to transcribe both sides.

What changes

• Adds a PocketStation capture worker that sends 16 kHz mono audio into MeetGraph's existing Segmenter and Segment queue.
• Keeps the current PortAudio microphone worker.
• Makes the existing system-device selector and PocketStation application selector mutually exclusive.
• Saves the selected application between launches.
• Reports missing installation, permission, and capture failures through the existing controller error signal.
• Keeps BlackHole, Stereo Mix, VB-Cable, and PipeWire monitor capture available and unchanged.

Verification

• 6 focused tests pass on Python 3.12.
• Wheel and sdist build successfully; neither contains generated bytecode.
• Real macOS 26.6.2 arm64 capture with PocketStation 0.1.3 from PyPI:
  • selected source: active afplay process
  • 29,760 samples reached MeetGraph's existing Meeting segment queue at 16 kHz
  • RMS: 0.08824
  • clean worker shutdown with no capture error

This is a real macOS application-audio result. Windows and Linux use the same published PocketStation API, but this PR does not claim a real MeetGraph run on those operating systems yet.

[tekrajchhetri]

Thanks @Raphjacksun7. Will review the PR.