Add “Show distinct values count” option to the status bar (similar to “Show selected row count”)

**Is your feature request related to a problem? Please describe.**
I often need to know how many distinct values a column has when exploring data. Currently I have to open Filter by value or the Calc panel every time, which is slower than just checking the status bar.

**Describe the solution you'd like**
Add a status-bar option similar to “Show selected row count”, but for “Show distinct values count” for the selected column. When a column is selected, the status bar would show its distinct value count.

**Describe alternatives you've considered**
- Using Filter by value with Show distinct values count
- Using the Calc panel
- Running COUNT(DISTINCT ...) queries manually

**Additional context**
This is mainly for quick data exploration and checking column cardinality without leaving the result grid.