feat(usage): export filtered usage records

[TWJohnJohn20116]

Summary

• add native save dialogs for CSV and JSON usage-history exports
• export every record matching the active time, model, provider, source, API key, and result filters
• make CSV output Excel-friendly with UTF-8 BOM and RFC-style escaping
• add localized export states and backend coverage for filtering, formatting, and file extensions

Testing

• bun run check
• bun test (111 passed)
• bun run build
• cargo test --manifest-path src-tauri/Cargo.toml (205 passed)