chore(site): swap AI add-on table icons (em-dash + plain check) (#26467)

Updates the icons in the **AI add-on** column for the admin tables on
`/deployment/users` and `/organizations/.../members`.

- Not consuming a seat: `XIcon` → muted em-dash
(`text-content-disabled`).
- Consuming a seat: `CircleCheckIcon` → plain `CheckIcon` from
`lucide-react` (no circle, still `text-content-success`).

Both tables share `site/src/modules/users/AISeatCell.tsx`, so the
single-file change covers `UsersTable.tsx` and
`OrganizationMembersTable.tsx`.

## Screenshot

Users admin table on a local dogfood build of this branch
(`/deployment/users`):

<!-- Drag-and-drop the screenshot here to attach it to the PR. -->

<details>
<summary>Notes</summary>

- The em-dash is rendered via the JSX entity `&mdash;` rather than a
literal `U+2014` character, so it does not trip `make lint/emdash` (see
`AGENTS.md` / `scripts/check_emdash.sh`). The browser still renders an
em-dash.
- Kept the existing `aria-label`s (`Consuming AI seat` / `Not consuming
AI seat`) and added `role="img"` on the em-dash span so screen readers
still announce the state.
- No callers other than `UsersTable.tsx` and
`OrganizationMembersTable.tsx` use `AISeatCell` (verified via search).

</details>

> Opened by Coder Agents on behalf of @tracyjohnsonux.

site/src/modules/users/AISeatCell.tsx

@@ -1,4 +1,4 @@
-import { CircleCheckIcon, XIcon } from "lucide-react";
+import { CheckIcon } from "lucide-react";
 import type { FC } from "react";
 import { TableCell } from "#/components/Table/Table";
 
@@ -10,15 +10,18 @@ export const AISeatCell: FC<AISeatCellProps> = ({ hasAISeat }) => {
 	return (
 		<TableCell>
 			{hasAISeat ? (
-				<CircleCheckIcon
+				<CheckIcon
 					className="size-5 text-content-success"
 					aria-label="Consuming AI seat"
 				/>
 			) : (
-				<XIcon
-					className="size-5 text-content-disabled"
+				<span
+					role="img"
 					aria-label="Not consuming AI seat"
-				/>
+					className="text-content-disabled"
+				>
+					&mdash;
+				</span>
 			)}
 		</TableCell>
 	);