Refactor configstore.h formatting and API

- Fix indentation and brace style to be consistent throughout
- Replace `Global::Instance().SetUtcOffsetIfValid()` with `global::SetUtcOffsetIfValid()`
- Add explicit casts for DEBUG_PRINTF format arguments to suppress warnings
- Expand single-line while loop bodies to multi-line for clarity
- Remove redundant `private:` section separator

Author: vanvught