refactor(config): inline validate.rs into settings.rs and remove settings/ folder

[StudentWeis]

Summary

Flatten the config/settings/ subdirectory by inlining the ~30-line validate.rs into settings.rs. No behavior change — purely structural cleanup.

Linked Issue

Closes #122

Changes

• Moved validation methods (validate_hotkey, validate_window_opacity, validate_storage) from src/config/settings/validate.rs into src/config/settings.rs
• Removed mod validate; declaration and the settings/ directory
• Added str::FromStr to the existing std import (was previously brought in by the submodule)
• Removed redundant use std::str::FromStr in the test module

Testing

• scripts/precheck.sh passes locally
• All 434 tests pass, including 38 settings-specific tests

Self-Check

• PR title follows Conventional Commits
• No hardcoded user-facing strings — i18n keys added to all locale files
• Errors defined with thiserror (no manual impl Display/Error)
• No unrelated changes mixed in