[slkass]

Adds 简体中文 (Simplified Chinese) to the language picker and shared interface text used by the window and tray menu. The translation covers all 413 Text fields (447 strings including button arrays), including controller setup, mappings and macros, profiles, lighting, gyro and stick settings, driver requirements, updates, and problem-reporting UI.

The language is saved as zh-cn, with zh and zh-CN accepted when reading settings. OPEN_CONTROLLER_LANG=zh-cn also selects Chinese for window and screenshot overrides. The README lists the new language, and English remains the default for new installations.

Validation on Windows 11 x64:

• cargo fmt --all --check
• cargo test --workspace --release --locked — 240 passed, 5 ignored
• cargo clippy --workspace --all-targets --release --locked -- -D warnings
• cargo build --release --locked
• Checked every translated field for matching placeholder counts and button-array lengths against English.