firmware: add OLED status panel to camera node (C6)

Bring up the 1.3" SH1106 128x64 I2C OLED on the XIAO ESP32-C6
(D4/SDA, D5/SCL @ 0x3C) per the Notion wiring diagram.

- add U8g2 dependency to the seeed_xiao_esp32c6 env
- I2C bus scan at boot (logs responders to serial)
- boot splash + live status screen: camera id, IDLE/REC + session
  timer, battery (raw until calibrated) + video-remaining, hub link
  state (MQTT/wifi/offline), and camera reachability
- refresh runs at the top of loop() so the panel stays live even
  when WiFi/MQTT are down

Verified on hardware: I2C scan finds 0x3C, U8g2 begin ok, panel
shows clean readable text.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/platformio.ini

@@ -65,7 +65,9 @@ platform = https://github.com/pioarduino/platform-espressif32/releases/download/
 board = seeed_xiao_esp32c6
 framework = arduino
 monitor_speed = 115200
-lib_deps = ${common.lib_deps}
+lib_deps =
+    ${common.lib_deps}
+    olikraus/U8g2 @ ^2.35
 build_flags = ${common.build_flags}
     -D CONFIG_ARDUINO_LOOP_STACK_SIZE=8192
     -D ARDUINO_USB_MODE=1