Settings: merge stepsToPerform and useBlockStepping into UI refresh rate option

[devin-ai-integration]

Settings: merge stepsToPerform and useBlockStepping into UI refresh rate option

Summary

Implements issue #410 by combining the separate "Number of batched steps" and "Block Stepping" settings into a single "UI refresh rate" dropdown option. The new setting provides two modes:

• "After N instructions": Allows configuring a custom number of instructions (with number input)
• "After block": Steps block-by-block (equivalent to the old block stepping behavior)

Key Changes:

• Added new UiRefreshRate type and Redux state structure
• Replaced two separate UI controls with a combined dropdown + conditional number input
• Updated getStepsToPerform() logic to use the new state structure
• Maintained backward compatibility by syncing old state fields when new setting changes

Review & Testing Checklist for Human

🔴 HIGH PRIORITY - I encountered testing limitations that require human verification:

• Test settings dialog functionality: Open the debugger settings dialog and verify the new "UI refresh rate" control appears and functions correctly (I was unable to open the settings dialog during testing)
• Test stepping behavior: Verify that both "After N instructions" and "After block" modes work correctly when running/stepping through programs
• Test UI interactions: Ensure the number input appears/disappears correctly when switching between dropdown options, and accepts valid numeric input (1-1000)
• Test backward compatibility: Check that existing debugger state doesn't break - try loading programs that previously used different combinations of the old settings
• Test edge cases: Invalid input values, switching between modes while debugging, etc.

Notes

⚠️ Testing Limitation: I was unable to fully test the UI functionality locally due to the settings dialog not opening properly in my browser session. The code changes follow the established patterns in the codebase, but the UI integration needs human verification.

The implementation maintains the existing logic flow - getStepsToPerform() now reads from uiRefreshRate instead of the separate fields, and the new setUiRefreshRate action keeps the old state fields in sync for any remaining dependencies.

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Requested by: @tomusdrw
Devin session: https://app.devin.ai/sessions/967237ffe34443caa7e18a1d87574ddf

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