Server mode writes no log file at all #291

Location: config.py:69
Severity: HIGH

LOGS_DIR is created (portable mode) but nothing in Python ever writes to it — logging goes to stdout only. Docker users get logs only if they capture them; source-run users get a scrollback buffer.
Add a RotatingFileHandler → LOGS_DIR/stemdeck.log (e.g. 5 × 5 MB) with a timestamped formatter, alongside stdout. One function in main.py.

From the health report. Part of #272 (Phase 1 — Evidence).