## Summary

Fixes arithmetic underflow panics in `terminal_scrollbar.rs` by
converting unsafe subtractions to `saturating_sub`.

Closes #45281

## Problem

Two locations perform raw subtraction on `usize` values that panic when
underflow occurs:

- `offset()`: `state.total_lines - state.viewport_lines -
state.display_offset`
- `set_offset()`: `state.total_lines - state.viewport_lines`

This happens when `total_lines < viewport_lines + display_offset`, which
can occur during terminal creation, with small window sizes, or when
display state becomes stale.

## Solution

Replace the two unsafe subtractions with `saturating_sub`, which returns
0 on underflow instead of panicking.

Also standardizes the existing `checked_sub().unwrap_or(0)` in
`max_offset()` to `saturating_sub` for consistency across the file.

## Changes

- N/A