fix: static theme blink, USB scan while connected, startup log

- Add TRCC version log on startup
- Fix static theme blink: keepalive() resends frame every ~20 s via
  mediator (period=20) to prevent USB standby when no dynamic overlay
  elements trigger a send
- Fix overlay_tick guard: was is_overlay_enabled|is_background_active,
  now device.connected — simpler and correct for all theme types
- Skip find_lcd_devices() sysfs scan when device already connected;
  poll only runs when no device is active
- ci.yml: safe if-then form for badge sed commands ([ -n ] exits 1 under bash -e)
- cli: uniform log format with timestamps across all verbosity levels

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>