Countdown display becomes garbled after terminal is resized by tiling WM

## Description

When `countdown` is running in Ghostty and I open several additional terminal windows, niri retiles and resizes the original terminal. After the resize, the large countdown digits become garbled/compressed near the bottom of the terminal instead of being redrawn legibly.

This looks related to #14, which was closed as completed, so this may be a regression or an unhandled small-terminal edge case.

## Steps to reproduce

1. Start Ghostty under niri.
2. Run a countdown, for example: `countdown 25s`.
3. Open several additional Ghostty terminal windows so niri tiles them and resizes the terminal running `countdown`.
4. Observe the countdown after the resize.

## Expected behavior

The timer should redraw cleanly within the new terminal dimensions. If the terminal is too small for the large digits, it should use a readable fallback or otherwise avoid corrupted output.

## Actual behavior

The digits are rendered as overlapping/garbled blocks and the timer becomes unreadable.

## Environment

- `countdown`: v1.5.0 (installed Go module version)
- Window manager: niri 26.04 (`8ed0da4`)
- Terminal: Ghostty 1.3.1-arch2.1

## Additional context

The previous resize issue was #14. The current code handles `termbox.EventResize`, but the result shown here is still corrupted when tiling causes the terminal to become substantially smaller.