Fix crash when stored playground language state is invalid

[martin-henz]

AI-generated PR, fixing #3645

Generated with Claude Code, Sonnet 4.6.

Description

Validate the stored chapter+variant combination against ALL_LANGUAGES when loading from localStorage. Falls back to the default language config if the combination is invalid (e.g. chapter 1 + explicit-control).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Code quality improvements

How to test

Start frontend using localhost, switch to conductor in feature flags, choose Source 1. Then reload in the browser. This would crash the frontend when the feature flag is reset to false, because the "Source 1" language does not have a CSE machine implementation.

Checklist

• I have tested this code
• I have updated the documentation

[RichDom2185]

This works but behaves differently than what the issue suggests.

[coveralls]

Coverage Report for CI Build 24697879728

Coverage increased (+0.02%) to 40.826%

Details

• Coverage increased (+0.02%) from the base build.
• Patch coverage: 5 of 5 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	13827
Covered Lines:	6098
Line Coverage:	44.1%
Relevant Branches:	7010
Covered Branches:	2409
Branch Coverage:	34.37%
Branches in Coverage %:	Yes
Coverage Strength:	28.82 hits per line

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💛 - Coveralls

[martin-henz]

This works but behaves differently than what the issue suggests.

In what way is this different from what the issue suggests?

[RichDom2185]

This works but behaves differently than what the issue suggests.

In what way is this different from what the issue suggests?

The issue suggests:

invalid combination of lang + variant → fallback to default lang + variant combination

But the logic implemented is:

invalid lang → fallback to default lang && invalid variant → fallback to default variant

[martin-henz]

This works but behaves differently than what the issue suggests.

In what way is this different from what the issue suggests?

The issue suggests:

invalid combination of lang + variant → fallback to default lang + variant combination

But the logic implemented is:

invalid lang → fallback to default lang && invalid variant → fallback to default variant

Hmm, the implementation checks:

          const validCombination = ALL_LANGUAGES.some(
            lang => lang.chapter === chapter && lang.variant === variant
          );

so it looks for a valid combination. It falls back on the default lang/variant only if the combination is not valid.