Active layer persists after map theme change - should activate first layer of new theme

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**Description**
When user changes the map theme in Mergin Maps mobile app and starts recording new features, the active layer from the previous theme remains active. This happens even if the current theme does not contain the layer (layer is inactive). After switching to another map theme, the first layer in the list of layers should be activated for recording.

**Environment**
Staging

**Application (+ app version, build, operating system)**
- Plugin version: 2025.3.1
- Android version: Android 14
- iOS version: iOS 18.5

**Steps to reproduce**
1. Open Mergin Maps app
2. Open project tester-ws-1/tc04_recording
3. Choose map theme "Paths and polygons"
4. Record polygon in PolygonLayer
5. Change map theme to "Aerial-points"
6. Record new point -> active layer is PolygonLayer

**Actual results**
- Active layer from previous theme (PolygonLayer) remains selected after switching to new map theme ("Aerial-points")
- User can record features using a layer that may not be visible or active in the current theme
- Layer selection does not automatically update when changing map themes

**Expected results**
- After switching to another map theme, the first layer in the list of layers should be automatically activated for recording
- Active layer selection should be updated to match the new theme's layer configuration
- User should not be able to record on layers that are inactive in the current theme

**Screenshots or logs**
Reference project: tester-ws-1/tc04_recording
Test performed on both Android and iPhone devices in staging environment

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

Reproduce the issue in tester-ws-1/tc04_recording by switching from “Paths and polygons” to “Aerial-points” and starting a recording. Trace the map-theme change and active-layer selection entry points, then verify that the first layer in the new theme is selected and inactive layers cannot receive new features on Android and iOS.

Written by the indexing model from the issue text.