Improve tenant slug field

[rchlfryn]

Description

The tenant slug drop down was a confusing UX because it displayed names first rather than the slug (which is the actual selection). This PR improves the tenant slug field UX in the admin panel. The slug is now displayed in uppercase throughout (list view and edit view), and is read-only on existing tenants since it should never change after creation.

Related Issues

Key Changes

• SlugCell: New custom cell component that renders the slug in uppercase in the list view
• TenantSlugField: On existing tenants, renders a read-only SelectInput showing the slug in uppercase. On new tenants, filters out already-used slugs from the dropdown as before. (bug introduced in Tenant improvements on addition and ability to delete #997)
• Slug option labels: Changed format from "Name (slug)" to "SLUG — Name" to emphasize the slug value

How to test

2. Verify slugs display in uppercase in the list view
3. Open an existing tenant — the slug field should be read-only and display the slug in uppercase (e.g., "NWAC")
4. Create a new tenant — the slug dropdown should only show unused slugs, with labels like "NWAC — Northwest Avalanche Center"

Screenshots / Demo video

When choosing

Before save

After save

Migration Explanation

No migration needed — these are admin UI display changes only.

[github-actions]

Preview deployment: https://change-slug.preview.avy-fx.org