Add sortable 'Added to Fleet' column to hosts table (#50098)

**Related issue:** Resolves #50083

# Screenshot demonstrating the fix

- Hosts page:
<img width="1473" height="335" alt="image"
src="https://github.com/user-attachments/assets/2f88ad6e-a514-4304-ac80-f56678e6be47"
/>

- Edit columns modal:
<img width="798" height="707" alt="image"
src="https://github.com/user-attachments/assets/0a79e8f3-1b21-4f84-bd04-726f44b9a0fa"
/>

## Note on sort direction

The new "Added to Fleet" column renders as a "days ago" duration (same
formatter as Last seen / Last fetched / Last restarted). To keep
behavior consistent across all four time-ago columns on the hosts table,
this PR applies the sort-direction inversion originally introduced for
`last_restarted_at` in #14878 (fix for #13160) to all of them:

- `seen_time` (Last seen)
- `detail_updated_at` (Last fetched)
- `last_restarted_at` (Last restarted) — unchanged behavior
- `last_enrolled_at` (Added to Fleet) — new

Arrow-down on any of these columns now sorts by the visible duration
(biggest "days ago" first / oldest date first), rather than by the raw
underlying timestamp. This is a user-facing behavior change on Last seen
and Last fetched — please re-QA sort order on those two columns
alongside the new one.

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

## Testing

- [x] QA'd all new/changed functionality manually

<!-- Attach: Edit columns modal showing the new "Added to Fleet" column
option, and the Hosts page with the column enabled -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
  * Added an **Added to Fleet** column to the hosts table.
* Displays when each host last enrolled with Fleet, with an explanatory
tooltip.
* The column is hidden by default and can be enabled through table
settings.
  * Supports ascending and descending sorting.

* **Bug Fixes**
* Corrected descending sorting for **Last seen** and **Last fetched** to
reflect the displayed host age.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: RachelElysia <71795832+RachelElysia@users.noreply.github.com>

changes/50083-added-to-fleet-column

@@ -0,0 +1,2 @@
+* Added a sortable "Added to Fleet" column to the hosts table, showing when each host last enrolled with Fleet.
+* Fixed sort direction on the "Last seen" and "Last fetched" columns of the hosts table so that sort descending puts the oldest date (biggest "days ago" number) first, matching the visible duration rather than the underlying timestamp.