Fix: search_projects crash on unknown category/difficulty values

[kumudasrip]

📝 Description

Fixed the registry display crash in search_projects.py caused by unknown or invalid category/difficulty values. The project badge rendering now uses safe .get(..., "Unknown")fallbacks, and list sorting now tolerates unsupported difficulty keys. I also hardened main.py display logic to avoid similar failures when registry entries contain unexpected values.

🔗 Linked Issue

Closes #1012

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📋 Contribution Checklist

• My code strictly adheres to the project guidelines (Standard library only, procedural code, clean spacing).
• I have verified that my files are placed in the correct directory.
• I have tested my changes thoroughly on my local machine.
• I have included interactive emojis and clean console/UI outputs.
• GSSoC 2026: I have been formally assigned to this issue and noted it above.

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[github-actions]

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