Range Slider with no min/max (±DBL_MAX) displays 2000000000 and slider does not respond

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**Description**

When a Range Slider field has no min/max configured in QGIS, QGIS automatically sets the boundaries to `±DBL_MAX` (`±179769313...858368`). The mobile app fails to handle this value and displays `2000000000` (or a truncated variant depending on precision) regardless of the actual field value. The slider handle cannot be moved.

The issue affects all unlimited range sliders regardless of step or precision. Sliders with explicit min/max (e.g. `-10` to `10`) are not affected and work correctly.

Note: QGIS itself also shows *"Current minimum for this value is 0 and current maximum is 0"* for these fields, which may be a related upstream issue.

**Environment**
[ ] Staging [x] PROD

**Application (+ app version, build, operating system)**
- Mergin Maps Mobile App — iOS 2025.6.0, iOS 26.0

**Steps to reproduce**
1. Open project `tester-ws-1/tc05_decimals` on staging
2. Create a new feature and navigate to the **Range Slider - Min/Max** tab
3. Move the slider in the widget with `Min: - Max: - Step: 1 Prec: 0` and observe the displayed number

**Actual results**
The slider displays `2000000000` and the number does not change when the slider is moved.

**Expected results**
The app should handle `±DBL_MAX` gracefully — either by clamping to a sensible default range, or by correctly reading and updating the field value.

**Screenshots or logs**

*Conclusion*
*Created by Claude and supervised by a human test analyst.*

Reproduce the issue in project tester-ws-1/tc05_decimals using the “Range Slider - Min/Max” tab and the unlimited slider configuration. Trace the mobile range-slider handling of QGIS ±DBL_MAX values and verify the resulting field value and handle movement. Done means unlimited sliders display and update a usable value, while sliders with explicit bounds continue to work.

Written by the indexing model from the issue text.