fix: prevent potential panic in search navigation with empty results

[echobt]

This PR fixes a potential race condition/panic in the TUI navigation logic.

Changes:

• Used saturating_sub(1) instead of direct subtraction for results.len() - 1 in next() and previous() methods.
• This ensures that even if results becomes empty between the initial check and the access (though unlikely in single-threaded contexts, it hardens the code against future changes or edge cases), the index calculation will not panic due to underflow.

Verification:

• Verified via code inspection that logic handles len() == 0 safely (returning 0, which is handled gracefully by Ratatui or subsequent logic).
• Ran existing tests (cargo test) to ensure no regressions.