[dustyghost]

Summary

Cleans up the app’s fullscreen/keybinding flow.
Adds full screen key

Changes

• make the fullscreen toggle configurable in settings instead of hard-coded
• keep old settings.toml files working by defaulting missing toggle_full_screen to F
• prevent duplicate key selections in the settings UI
• refactor app.rs into smaller input/render helpers
• add regression tests for settings compatibility and key filtering

Validation

• cargo test passes locally 11/11