feat: add safer restore modes

[dinakars777]

Summary

Adds safer restore choices to the TUI so users are not forced through git reset --hard for every recovery path. Enter remains the hard-reset flow, but it now creates a backup ref first; s performs a soft reset, and c checks out the selected commit in detached HEAD.

The confirmation dialog now shows the exact Git command before execution, and full diff preview now compares HEAD against the selected target so the preview matches restore semantics instead of showing only the selected commit's patch.

Closes #17.

Verification

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo build
• git diff --check
• cargo run -- --help