🧪 [testing improvement] Add UI test for spawn_settings_panel

[dynamikdev]

🎯 What: The testing gap addressed
The issue asked to implement a test for spawn_settings_panel located in src/ui/settings.rs:21. Bevy UI tests are a bit complex since they require an App set up with plugins to verify spawned entities instead of simple functional outputs.

📊 Coverage: What scenarios are now tested
A test test_spawn_settings_panel is implemented in src/ui/settings_test.rs which verifies:

• Initialization and creation of the App with MinimalPlugins and SliderPlugin
• Insertion of necessary global resources (Typography)
• Proper execution of the spawn_settings_panel system logic
• Detection of all required main headers and button texts, ensuring the Settings Panel Title, Sequence Mode Button, Sequence Control Button, Rhythm Setup, and Curriculum elements all spawn accurately.

✨ Result: The improvement in test coverage
The code base now has dedicated regression testing ensuring that if anyone breaks or removes key elements of the configuration sidebar (spawn_settings_panel), tests will fail immediately, adding much-needed safety.

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PR created automatically by Jules for task 6366564029895385069 started by @dynamikdev

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