Value Relation (multi-select) — no way to clear all selected values or filter by selected items on mobile

Summary
When a Value Relation field is configured with Allow multiple selection enabled and the list of available values is large (e.g. 500+ items), there are two missing interactions that make managing selected values impractical on mobile:
A. There is no mechanism to clear all currently selected values in one action
B. There is no way to filter the list to show only the currently selected items.

A. Steps to reproduce — Clear all

1. Configure a layer with a Value Relation field, enable Allow multiple selection, and ensure the referenced table contains 500+ records / or use the Project: tester-ws-1/tc05_value_select
2. Open the feature form on mobile and select ~100 values in that field - Visited cities field
3. Save the feature. Reopen the form.
4. Attempt to clear all selected values.

B. Steps to reproduce — Filter by selected

1. Same setup as above, with ~100 values already selected.
2. Reopen the feature form and open the Value Relation field - Visited cities field
3. Attempt to filter or view only the 100 currently selected items to review or modify them.

Expected behaviour

A Clear all (or Deselect all) action should be available — for example, a button or long-press context menu — allowing the user to remove all currently selected values in a single interaction.
A Show selected only filter option should be available, allowing the user to narrow the visible list to only the items currently selected, making it easy to review and manage the active selection without scrolling through hundreds of entries.

Actual behaviour

No Clear all action exists. The user must scroll through the full list and tap each selected item individually to deselect it.
No Show selected only filter exists. Selected items are scattered across the full list with no way to isolate them, making it very difficult to review what is currently selected when the list is large.

Start by reproducing the Value Relation field behavior in Project: tester-ws-1/tc05_value_select on a mobile form with about 100 selected values. Inspect the mobile Value Relation field implementation and its selection controls; done means users can clear all values and filter the list to show only selected items.