Refactor: eliminate frequency-management code duplication between main.cpp and FrequencyManager

[genestealer]

Problem

The standalone MQTT firmware (src/main.cpp) contains its own inline re-implementation of every frequency-management concern that was subsequently extracted into src/services/frequency_manager.cpp / FrequencyManager. This means the same logic is maintained in two places and they can silently diverge.

Duplicated symbols

main.cpp (inline)	src/services/frequency_manager.cpp (service class)
FEED_WDT() static helper	FrequencyManager::feedWatchdog()
saveFrequencyOffset(float) raw EEPROM/Preferences	FrequencyManager::saveFrequencyOffset(float) via StorageAbstraction
loadFrequencyOffset() raw EEPROM/Preferences	FrequencyManager::loadFrequencyOffset()
performDeepFrequencyScan(float, float) identical two-phase algorithm	FrequencyManager::performDeepFrequencyScan(float, float, statusCallback)
adaptiveFrequencyTracking(int8_t)	FrequencyManager::adaptiveFrequencyTracking(int8_t)
Globals: storedFrequencyOffset, autoScanEnabled, cumulativeFreqError etc.	Statics: s_storedOffset, s_autoScanEnabled, s_cumulativeFreqError etc.

Notable divergence already present

• main.cpp performDeepFrequencyScan calls cc1101_init() and get_meter_data() directly; FrequencyManager uses injected callbacks.
• main.cpp storage layer uses raw EEPROM/Preferences; FrequencyManager uses the StorageAbstraction wrapper.
• FREQEST_TO_MHZ constant is defined locally in main.cpp; service class exposes it as a named constant.

Desired outcome

main.cpp should delegate all frequency-management operations to FrequencyManager:

1. Register cc1101_init and get_meter_data as callbacks via FrequencyManager::setRadioInitCallback / setMeterReadCallback.
2. Call FrequencyManager::begin(FREQUENCY) during setup instead of loadFrequencyOffset().
3. Replace inline definitions in main.cpp with calls to the corresponding FrequencyManager static methods.
4. Remove duplicate global frequency state variables from main.cpp; read from FrequencyManager::getOffset() / getTunedFrequency().

Files to change

• src/main.cpp - remove inline implementations, wire up FrequencyManager
• src/services/frequency_manager.h / .cpp - ensure public API covers all current main.cpp inline use
• src/services/storage_abstraction.h - already abstracts EEPROM vs Preferences; main.cpp should use this