Set up automated style enforcement (linting/formatting)

[hurui200320]

The style rules documented in STYLE_GUIDE.md are currently enforced only by code review.

Tasks:

• Kotlin (android/): consider ktlint and/or spotless.
• C++ (esp32/): consider clang-format with the existing style (PascalCase files/namespaces, camelCase methods, UPPER_SNAKE_CASE macros) in mind.
• Decide where to run the checks: pre-commit hook, CI job, or editor integration.
• If CI is used, decide how it interacts with the release CI tracked in Set up release CI or F-Droid build & distribution #12 (e.g., shared workflow files).
• Update STYLE_GUIDE.md's Enforcement section once a decision is made.