Fix selected layers in screenshot

[PartyNell]

Description

When taking a screenshot of the map, some layers were not correctly displayed.
This happened because the user interface language (e.g. French, Spanish, etc.) differed from the language used by the headless browser in Screamshotter, which defaults to English. As a result, the application context could not be properly restored due to the language mismatch.
This PR introduces a new lang parameter in the application URL.
It allows Screamshotter to explicitly specify the language to use when loading the application, ensuring that the context is restored consistently across languages.

Related Issue

• Screenshot de la carte : prendre en compte les filtres et les éléments cochés dans les couches GeotrekCE/Geotrek-admin#4630

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.84%. Comparing base (fdb4728) to head (4100e9a).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   90.94%   91.84%   +0.90%     
==========================================
  Files          33       33              
  Lines        2452     2453       +1     
==========================================
+ Hits         2230     2253      +23     
+ Misses        222      200      -22     

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