feat(firmware): log reason before FATAL hang loops

Two FATAL while(true) hangs in main.cpp (config load fail, camera init
fail) previously relied on the hardware watchdog to reboot the device,
leaving the cause invisible beyond a generic TWDT reset reason. Now
each path logs EVT_REBOOT with REBOOT_FATAL_CONFIG or REBOOT_FATAL_CAMERA
before hanging, so the next heartbeat's recent_events surfaces which
branch hung. Server-side decoder updated for the two new enum values.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/heartbeat_diagnostics_stub.py

@@ -122,4 +122,6 @@ REBOOT_REASON_DECODER = {
     2: "FACTORY_RESET",
     3: "OTA",
     4: "WIFI_REPROV",
+    5: "FATAL_CONFIG",
+    6: "FATAL_CAMERA",
 }