TESAIoT PSE84 Programmer v0.2.4 — push board_info on USB hotplug

Adds proactive board_info push over the Remote Flash WebSocket on every USB hotplug + an 8s heartbeat. Fixes a relay-team-reported staleness where the Web peer kept showing the kit as Connected after the operator unplugged it. Spec match: {type:"board_info", ts, payload:{board_connected, serial, device_mpn, security_class, kit_name, display_name}}. Same install rules as v0.2.3 (macOS first-launch right-click → Open; Windows SmartScreen 'More info' → 'Run anyway').