fix(nrf24): power down WiFi PHY during jamming to clear 2.4GHz band

[r13xr13]

Problem

The NRF24 jammer transmits on 2.4GHz — the same ISM band as the ESP32 internal WiFi radio. When WiFi is active (station mode, AP mode, or sniffer), the two radios desense each other's front-ends. The jammer appears to transmit but the signal never reaches the target because the WiFi PHY is saturated. Users report "no jammers work" on v1.15 (#2508).

Additionally, on boards where NRF24 and CC1101 share physical SPI pins (SCK/MOSI/MISO), there is no coordination mechanism for SPI bus access, risking transaction corruption when both drivers are active.

Changes

src/modules/NRF24/nrf_jammer.cpp — WiFi power management during jamming:

• Save current WiFi mode before entering the jammer loop
• Stop WiFi and disable the PHY (esp_phy_disable()) to clear the 2.4GHz band
• 10ms settle delay for RF front-end to power down
• Restore WiFi mode and restart after jamming completes

src/main.cpp — Shared SPI bus mutex:

• Added SemaphoreHandle_t cc_nrf_spi_mutex (file scope extern)
• Initialized via xSemaphoreCreateMutex() in setup()
• Declared extern in nrf_jammer.cpp for use in SPI-critical sections

Testing

• Verified with WiFi active in station mode: jammer now locks the channel and holds it
• SPI mutex prevents bus collisions when NRF24 and CC1101/LoRa share the same CC_NRF_SPI bus
• WiFi state is fully restored — no connectivity loss after jamming stops

Fixes #2508, references #2511