Add Simple QWERTY scanning keyboard layout

[enaboapps]

Summary

• Add a saved Simple QWERTY keyboard layout with three scanned letter rows, predictions, and the existing footer.
• Keep removed punctuation and shifted forms on the Simple layout's Numbers page. Prose punctuation retains smart spacing and capitalization; the numeric decimal key remains literal.
• Offer the layout in settings and cycle QWERTY → Simple QWERTY → Common letters first → QWERTY from the footer.

Closes #962

Validation

• Node 24.19.0: npm run prediction-model, npm run lint, npm test (248 Vitest and 9 Node tests), and npm run build passed.
• Rust 1.97.1: cargo fmt --manifest-path src-tauri/Cargo.toml --check, cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings, and cargo test --manifest-path src-tauri/Cargo.toml passed (616 unit tests, 1 ignored, 7 integration tests). A final focused Simple QWERTY run passed 3 tests.
• Windows 11 Parallels VM: attempted native validation, but its screen stayed at 8:31 AM and ignored Windows key input; unable to build or exercise the branch there. No installed app was changed.