Experimental features visible after reinstall with "Show also experimental plugins" unchecked

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**Description**

Experimentalfeatures in Mergin Maps plugin are visible after a clean reinstall even though “Show also experimental plugins” is unchecked in Plugin Settings. This appears to be a regression of closed issue #724.

**Environment**
Staging

**Application (+ app version, build, operating system)**
- QGIS version: 3.40.5-Bratislava
- Plugin version: 2025.3.2
- OS: macOS, Sequoia 15.6

**Steps to reproduce**
1. Uninstall the Mergin Maps plugin.
2. Open Plugins -> Manage and Install Plugins… -> Settings; ensure “Show also experimental plugins” is UNCHECKED.
3. Install the Mergin Maps plugin (verify the checkbox remains unchecked in Settings after install).
4. Open Project -> Properties -> Mergin Maps.
5. Observe presence of experimental features (e.g. Photo sketching or other experimental options) that should be hidden.

**Actual results**
Experimental features are visible in Project Properties despite the experimental plugins option being disabled before installation and remaining unchecked afterwards.

**Expected results**
Experimental features should stay hidden whenever “Show also experimental plugins” is unchecked, including on fresh install/reinstall paths.

**Screenshots or logs**
(Not provided)

*Conclusion*
*Created by GitHub Copilot and supervised by a human test analyst.*

Start by reproducing the reinstall path in QGIS, checking Plugins → Manage and Install Plugins → Settings before and after installing the Mergin Maps plugin. Then open Project → Properties → Mergin Maps and compare visibility with “Show also experimental plugins” unchecked. Done means experimental features remain hidden after a fresh install or reinstall.

Written by the indexing model from the issue text.