Add UI filter to show watches without errors

## Summary

The watch list UI currently provides a "With errors" filter/count, but there does not appear to be an equivalent way to show only watches that do not have errors.

## Use case

After bulk-importing a list of URLs, I want to quickly separate:

1. Watches with errors that need debugging
2. Watches without errors that imported/check correctly

The current "With errors" filter helps with the first group, but there is no obvious inverse filter for the second group.

## Expected behavior

Add a UI filter such as:

- Without errors
- No errors
- Healthy
- OK

Ideally it would appear alongside the existing "With errors" filter.

## Actual behavior

The UI shows a "With errors" filter, for example "With errors (15)", but I cannot find a way to filter the list to only watches without errors.

## Why this would help

This would make post-import cleanup much easier, especially when managing a large watch list. Users could quickly review successfully configured watches without paging through all watches or using the API.

## Possible implementation

Reuse the inverse of the existing error-filter predicate used by "With errors".

For example:

- With errors: watches where the current error state is present
- Without errors: watches where that error state is not present

## Environment

- changedetection.io version:
- Install method: Docker / pip / other:
- Browser:

Start by locating the existing "With errors" filter and the error-filter predicate used by the watch list UI. Trace how its label and count are rendered, then add the inverse filter alongside it. Done means users can select the new filter and see only watches without current errors, including the correct count.

Written by the indexing model from the issue text.