Fix build warnings: guard syncword macros & fix narrowing conversions

[AbedKarmi]

Title: Fix build warnings: guard syncword macros & fix narrowing conversions

Summary

This patch fixes compiler warnings observed when building projects that use the Heltec ESP32 Dev-Boards library together with other radio/driver code. The changes are minimal and backwards-compatible:

• Guard LORA_MAC_PRIVATE_SYNCWORD and LORA_MAC_PUBLIC_SYNCWORD macros with #ifndef to avoid macro redefinition warnings when different values are defined in other headers (e.g., driver headers). This reduces spurious warnings when the library is included in larger projects.

• Add explicit casts to uint8_t in several spots in HT_st7735.cpp to avoid narrowing-conversion warnings on modern compilers with -Wnarrowing.

Why

These warnings were seen during builds and are noisy. They come from two sources:

1. Macro redefinitions (LoRa syncwords) across multiple headers (Harmless but noisy)
2. Narrowing conversions when initializing byte arrays from arithmetic expressions (compiler warns when moving from int to uint8_t)

Where

• src/loramac/LoRaMac.h - wrap macro definitions in #ifndef guards
• src/driver/sx126x.h - same guard updates
• src/HT_st7735.cpp - add (uint8_t) casts where appropriate

Testing

• Built LoRa-Alarm project that includes the forked Heltec library. Build shows no warnings originating from the Heltec files changed in this patch. Existing unit tests (if any) remain unaffected.

Notes

• Changes are minimal and should be safe for the mainline library.
• If maintainers prefer, I can instead add compiler-conditional guards or macros to keep original values for certain targets.

Suggested commit message

Fix build warnings: guard syncword macros & fix narrowing conversions

• Add #ifndef guards around LORA_MAC_PRIVATE_SYNCWORD and LORA_MAC_PUBLIC_SYNCWORD
• Fix narrowing conversion by casting values to uint8_t in HT_st7735.cpp

Commands to apply the patch locally and create a PR

1) Create a branch

git checkout -b fix/heltec-build-warnings

2) Apply patch (if you saved the patch file as heltec-fixes.patch in the repo root)

git apply heltec-fixes.patch

3) Commit

git add src/loramac/LoRaMac.h src/driver/sx126x.h src/HT_st7735.cpp
git commit -m "Fix build warnings: guard syncword macros & fix narrowing conversions"

4) Push to your fork and open a PR

(Assuming your fork remote is 'origin')

git push -u origin fix/heltec-build-warnings

Then open a PR on GitHub comparing fix/heltec-build-warnings to heltec master/main and paste the PR description above.

If you want, I can prepare a Git branch and push to your fork (requires your fork remote or GitHub token). Otherwise, you're ready to paste the patch and PR text in your fork's PR.