Dashboard table widgets: support percentage-of-total calculated columns

### Problem Statement

It would be useful to add a "percentage of total" column type to dashboard table widgets. Currently, table widgets only support raw aggregates (eg. `sum()`). There's no way to express a derived value like "this row's value as a percentage of the column total."

For example, I am making a dashboard to group AI spend by feature.

Raw numbers are helpful but it would be nice to have a percentage here. I sort of get this with a bar (time series) chart; I just want the same info in tabular form.

### Solution Brainstorm

_No response_

### Product Area

Dashboards

No files, tests, or entry points are named in the issue. Start by locating dashboard table widget column definitions and the handling for raw aggregates, then clarify how totals and sorting should behave with maintainers. Done means a table widget can display each row's share of the relevant column total, with coverage for the intended behavior.

Written by the indexing model from the issue text.