fix: add static_assert for BLE structs and [[nodiscard]] to methods

[gdellis]

Summary

Address issues #61 and #62:

• Add static_assert to verify BLE struct sizes at compile time
• Add [[nodiscard]] to all esp_err_t returning methods to catch ignored errors at compile time
• Fix state_machine.cpp to use ESP_ERROR_CHECK on ble_.send_alert() call (required after adding [[nodiscard]])

Changes

firmware/main/ble.hpp:

• Add #pragma pack(push, 1) and static_assert for BleLocationData (17 bytes) and BleAlertData (18 bytes)
• Add [[nodiscard]] to: init(), start(), stop(), update_location(), send_alert(), set_device_name()

firmware/main/state_machine.cpp:

• Use ESP_ERROR_CHECK on ble_.send_alert() in geofence breach handling

Testing

• Build passes for ESP32-S3
• Host tests pass

Closes #61, #62

[gdellis]

The changes this PR intended to add (static_assert for BLE structs and [[nodiscard]] on esp_err_t methods) were already merged via PR #52.

The branch has been rebased onto current main and no additional changes are needed. Closing as duplicate.