firmware: no-reflash config updates for ESP-01S + UART-OTA groundwork

Updating the buried ESP-01S currently means a USB-UART adapter and a
GPIO0 jumper. Add a path to change its settings without reflashing, and
lay the groundwork for full firmware updates over the existing UART.

set_config (no reflash for settings):
- ESP-01S: add saveConfig() + a set_config command — updates GoPro
  SSID/password/IP and poll interval, persists to LittleFS, acks, and
  re-associates Wi-Fi if creds changed
- XIAO: forward an MQTT set_camera_config down to the ESP-01S over UART
  (hub -> MQTT -> XIAO -> UART -> ESP-01S/LittleFS)

UART-OTA groundwork ("XIAO as flasher"):
- reserve XIAO GPIOs ESP01_RST_PIN=D8, ESP01_PGM_PIN=D10 for driving the
  ESP-01S serial bootloader (not driven yet)
- docs/design/esp01s-uart-ota.md: full design (why Wi-Fi OTA doesn't fit
  the 1MB ESP-01S on the GoPro AP, bootloader entry, ROM flash protocol,
  HTTP-pull delivery, scope)
- hardware/README.md: fix stale ESP32-C3 -> XIAO ESP32-C6 wiring, add the
  two control lines (Notion wiring diagram updated to match)

Both firmwares build clean and are flashed; set_config round-trip needs
the broker to exercise end-to-end.

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

hardware/README.md

@@ -129,7 +129,7 @@ USB Power Bank
   └── USB-C cable → bottom USB-C female input on Camera Node Case
           ├── rocker switch → node power rail
           ├── PWR LED indicator
-          ├── ESP32-C3 Super Mini
+          ├── XIAO ESP32-C6
           ├── ESP-01S / ESP8266
           ├── 1.3-inch OLED display
           ├── RGB status LED
@@ -138,9 +138,14 @@ USB Power Bank
                                           (power only — no data)
 
 UART / control inside case:
-  ESP-01S TX  ──→ ESP32-C3 RX
-  ESP-01S RX  ←── ESP32-C3 TX
-  ESP-01S GND ─── ESP32-C3 GND
+  ESP-01S TX (GPIO1)  ──→ XIAO D7 (RX)
+  ESP-01S RX (GPIO3)  ←── XIAO D6 (TX)
+  ESP-01S GND         ─── XIAO GND
+
+  # Reserved for UART OTA (XIAO reflashes the ESP-01S — no adapter):
+  ESP-01S RST         ←── XIAO D8   (pulse low to reset)
+  ESP-01S GPIO0       ←── XIAO D10  (low at reset = bootloader)
+  # See docs/design/esp01s-uart-ota.md
 ```
 
 **Power note:** exact wiring depends on the regulator/power board used. Confirm OLED voltage, LED current limiting, and whether the rocker switches USB 5 V input or a regulated node rail.