Enable Dropdown Expansion for Read-Only Value Relation and Value Map Widgets

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**Current Behavior:** when a Value Relation or Value Map widget is set to "not editable" in QGIS (either via field properties or an expression), the widget becomes completely static in Mergin Maps mobile app. The user can view the currently selected value but cannot interact with the widget to expand the dropdown or view the list of other potential values.

**Proposed Enhancement:** MM allow users to expand the dropdown list even when the field is set to read-only (in QGIS we can view the values even when a field is not editable)
- View-Only Access: Users should be able to open the dropdown to view the full list of available options (e.g., to understand the domain of values or see alternative categories).
- Prevent Selection: While the list is viewable, the user must remain unable to change or commit a new value to the field.

The issue names no files or tests. Start by locating the mobile implementations of Value Relation and Value Map widgets and how their read-only state is handled; done means a read-only widget can open its options list without allowing selection or committing a changed value.

Written by the indexing model from the issue text.