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Improves file safety and everyday keyboard navigation.
- Save files atomically and detect external changes before overwriting them.
- Focus the filename field with Ctrl+N and return to the new editor with Enter.
- Move to the end of a clean document with Ctrl+period.
- Show offline English word definitions with Ctrl+comma.
- Discard stale spelling suggestions and cancelled lookup results.
- Split application handlers into focused modules and clarify comments.
Validation: 102 tests passed; formatting and Clippy checks passed. Keyboard flows verified in the Linux app.