Selected users are buried in alphabetical order in filter dropdowns

### Description
In user filter dropdowns such as **“Assigned to…”**, **“Added by…”**, and **“Closed by…”**, selected users remain mixed with unselected users in alphabetical order. This makes it difficult to quickly see and manage the current selections, especially when many users are present.

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### Steps to reproduce
1. Open any board
2. Click on the **“Assigned to…”** filter
3. Select one or more users
4. Observe the order of items in the dropdown

Repeat the same steps for:
- **“Added by…”**
- **“Closed by…”**

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### Expected behavior
- Selected users should appear at the top of the dropdown (sorted alphabetically among themselves)
- Unselected users should appear below (also sorted alphabetically)
- Special options such as **“No one”** should remain pinned at the very top

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### Actual behavior
Selected users remain buried in the full alphabetical list and are hard to find once selected.

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### Impact
When many users exist, it becomes difficult to:
- See which filters are currently active
- Quickly unselect users
- Manage multiple selections efficiently