Fix model.split error when model is array

[bledden]

Summary

Fixes #117 - Frontend crashes when model is an array instead of a string.

Problem

The frontend code calls model.split('/') but some API responses return model as an array (e.g., ["openai/gpt-4o"]) instead of a string, causing:

TypeError: finalResponse.model.split is not a function

Solution

Added getModelDisplayName() helper in frontend/src/utils.js that handles:

• Arrays (extracts first element)
• null/undefined (returns "Unknown")
• Non-string types (converts to string)
• Missing slash (returns whole string)

Updated Stage1, Stage2, and Stage3 components to use this helper.

Validation

// BEFORE (crashes):
finalResponse.model = ["openai/gpt-4o"]
finalResponse.model.split('/') → TypeError: split is not a function

// AFTER (works):
getModelDisplayName(["openai/gpt-4o"]) → "gpt-4o"

Test plan

• Verify frontend doesn't crash when model is returned as array
• Verify normal string models still display correctly

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