Browser average interval/ease columns can overflow

### Problem

The Browser table calculates average interval by summing day intervals as `u32` and then multiplying by 86,400 in `u32`. A single interval above about 49,710 days, or multiple large intervals in notes mode, can overflow. The average-ease calculation similarly sums values as `u16`, which can overflow with roughly 27 cards at ease 2500 in notes mode.

In debug builds this can panic; in release builds it can display an incorrect value.

### Steps to reproduce

1. Give a card a very large interval, for example with Set Due Date, a high maximum interval, or an add-on.
2. Show Average Interval in the Browser.
3. Alternatively, select about 30 cards in notes mode with Average Ease shown.

### Expected behavior

The Browser computes and displays the averages without integer overflow.

### Additional context

I have a small fix with regression tests ready and will link it here.

Start with the Browser's Average Interval and Average Ease calculations, reproducing the issue with a very large interval or about 30 cards in notes mode. Review the regression tests mentioned in the issue and confirm that the Browser displays correct averages without debug-build panics or release-build overflow.

Written by the indexing model from the issue text.