Add MJPEG live mode dispatch, make --replay optional

[Konstantysz]

Summary

• --replay is now optional in Sh3DSAppGUI.cpp — omitting it triggers live camera mode
• BuildPipeline() has three-way dispatch:
  • --replay non-empty → existing FileFrameSource / VideoFrameSource (seeker set)
  • camera.type == "mjpeg" → MjpegFrameSource, seeker = nullptr (live mode sentinel)
  • Unknown type → LOG_ERROR + std::runtime_error
• Window title updated to "SH-3DS Debug — LIVE" or "SH-3DS Debug — Replay" based on seeker

Test plan

• All 169 tests pass (no regressions)
• --replay path → identical behavior to v0.1.0 (FileFrameSource / VideoFrameSource)
• camera.type: "mjpeg" without --replay → MjpegFrameSource created, seeker == nullptr
• Unknown camera.type → readable error + clean exception

Closes #6