refactor: migrate all mod.rs files to parent-named module files

[StudentWeis]

Summary

Migrate every src/<name>/mod.rs to the parent-named single-file form src/<name>.rs, satisfying the newly-denied clippy::mod_module_files lint. Purely mechanical rename — no behavior or API change.

Linked Issue

Closes #80

Changes

• Rename 13 mod.rs files (via git mv, history preserved):
  • src/clipboard/mod.rs → src/clipboard.rs
  • src/config/mod.rs → src/config.rs
  • src/config/settings/mod.rs → src/config/settings.rs
  • src/gui/mod.rs → src/gui.rs
  • src/gui/board/mod.rs → src/gui/board.rs
  • src/gui/board/records_list/mod.rs → src/gui/board/records_list.rs
  • src/gui/board/settings_handler/mod.rs → src/gui/board/settings_handler.rs
  • src/gui/panel/mod.rs → src/gui/panel.rs
  • src/i18n/mod.rs → src/i18n.rs
  • src/repository/mod.rs → src/repository.rs
  • src/repository/tests/mod.rs → src/repository/tests.rs
  • src/updater/mod.rs → src/updater.rs
  • src/utils/mod.rs → src/utils.rs
• Add mod_module_files = "deny" to Cargo.toml to keep this idiom enforced going forward.

Testing

• scripts/precheck.sh passes locally (clippy clean, 421 tests passed, i18n/icons/themes OK)
• No new tests needed — purely mechanical refactor with no behavior change