Structured file logging

[PyXiion]

Origin of idea

loguru logs to stderr only. No persistent log files. Debugging a user-reported issue requires them to reproduce while watching terminal output. LOGURU_LEVEL=DEBUG is set only in dev mode.

Suggestion / proposed solution

Add logger.add() in _app.py:

• Write to config_dir()/logs/pxmodrim.log with rotation (10 MB, keep 5)
• JSON serialization option for machine parsing
• Always write INFO+ to file; DEBUG to file when --debug flag or env var set
• Include process ID, timestamp, module:line in format
• Add "Open logs folder" action to Help menu
  Related: issue More useful logs #15 (More useful logs) for content; this issue is for infrastructure.

Alternatives considered

• Structured logging framework (structlog) — heavier, loguru already sufficient

Additional context

_app.py:28-55 has exception hooks wired to loguru. LOGURU_LEVEL=DEBUG in justfile.