Data table: transparent frozen column, misplaced Total rows footer, missing separator in actions menu

## Problem

On the Data Table page (`/automation/datatables/:id`):

1. **Frozen id column is transparent.** When the grid is scrolled horizontally, cell content of the other columns shows through the frozen row-number column. Rows, the header and the grid itself also have no background, and some column borders render black.
2. **"Total rows" footer is not in the first column.** It is rendered in the summary cell of the first data column instead of starting at the frozen row-number column.
3. **Missing separator in the table actions menu.** There is no separator between **Rename Table** and the destructive **Delete Table** item.

## Cause

1. `react-data-grid` defines its colors with `light-dark()`. Lightning CSS polyfills it into `var(--lightningcss-light, …) var(--lightningcss-dark, …)`, which only resolves when it can also rewrite a literal `color-scheme` declaration. The grid sets `color-scheme` through a variable, so `--lightningcss-light/dark` are never defined and every grid color computes to an invalid value (e.g. `--rdg-background-color: #fff#212121`). Frozen cells use `background-color: inherit`, so they end up transparent.
2. `useDataTable` renders `BottomLoader` in the first non-frozen column (spanning the rest), because a frozen column cannot span into unfrozen columns.
3. `DataTableActionsMenu` never rendered a `DropdownMenuSeparator`.

## Expected

The grid uses opaque, theme-aware colors; the "Total rows" footer starts in the frozen first column and stays visible when scrolling horizontally; **Delete Table** is separated from the other actions.