feat: vendor-neutral GUI editor naming

[gitkeniwo]

Summary

• introduce the gui_editor setting and snippet.open-gui command
• retain vscode_cmd and snippet.open-vscode as deprecated 0.x aliases
• resolve configured editor names in the TUI palette, help, search, and status messages

Verification

• cargo fmt --all --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo clippy --locked --all-targets --no-default-features -- -D warnings
• cargo test --locked --all-features
• cargo test --locked
• generated documentation checks