Add an interactive Ratatui TUI for foxguard triage

[peaktwilight]

Summary

• add an opt-in foxguard ui subcommand backed by Ratatui/Crossterm for interactive scan, diff, and secrets triage
• extract shared scan, diff, and secrets execution into src/app.rs and collect structured notices so the TUI and CLI can reuse the same flows
• add TUI usability improvements including severity/search filters, notices panel, elapsed scan timing, rescan safety, and editor shortcuts for opening the selected finding

Testing

• cargo fmt
• cargo test
• cargo run --bin foxguard -- ui --help
• printf 'q' | script -q /dev/null target/debug/foxguard ui .