Integrate new Vue mirror table

What does this MR do and why?

Contributes to #595303

Problem

The final step of the Haml to Vue migration for mirror repositories wires the MirrorTable component into the repository settings page.

Solution

• Wire mountMirrorTable into the CE and EE repository settings pages behind the vue_mirror_table feature flag, keeping the legacy HAML table when the flag is disabled.
• Add frontend, helper, and view specs covering the new path.

References

Add enabled/disabled toggle for push mirrors in... (#595303)

Screenshots or screen recordings

Before	After

How to set up and validate locally

This change is behind the vue_mirror_table feature flag (disabled by default).

1. Enable the feature flag in a Rails console:

   Feature.enable(:vue_mirror_table)

2. Go to a project's Settings > Repository and expand the Mirroring repositories section.
3. Add one or more push mirrors (and, on EE with the repository_mirrors license, a pull mirror) and confirm the new Vue table renders with:
   • Repository URL, Direction, Last update attempt, Last successful update, Status, and Actions columns populated correctly (timestamps show a relative time, not "Never", once a sync has run).
   • On EE: the branch-settings badge appears when a branch filter is set, and the pull mirror row appears with the correct direction.
4. Exercise the row actions and confirm they work and reflect state:
   • Update now triggers a sync.
   • Disable/Enable toggles a push mirror.
   • Remove deletes the mirror (deleting the pull mirror re-enables the mirror direction selector in the form).
5. Verify the empty state ("There are currently no mirrored repositories.") shows when no mirrors exist.
6. Disable the flag (Feature.disable(:vue_mirror_table)) and confirm the legacy HAML table still renders unchanged.

MR acceptance checklist

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