🧪 add tests for rhythm_mode_toggle_system

[dynamikdev]

🎯 What: The testing gap addressed
The system rhythm_mode_toggle_system in src/ui/systems.rs was previously missing automated tests for its logic covering state changes and interaction feedback.

📊 Coverage: What scenarios are now tested

• Tested Interaction::Pressed transitioning RhythmState::mode from Constant to Accelerate and verifying text output.
• Tested Interaction::Pressed transitioning from Accelerate to Constant and verifying text output.
• Tested Interaction::Hovered updates the background color correctly for user feedback without changing state.
• Tested Interaction::None resets the background color correctly.

✨ Result: The improvement in test coverage
The critical behavior of toggling sequence rhythms is now explicitly verified via unit tests, enabling future refactors to occur confidently without silently breaking standard app behaviors.

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PR created automatically by Jules for task 6734105069151792081 started by @dynamikdev

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