Add SecuSpark - free gamified CompTIA cert prep

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What is SecuSpark?

SecuSpark is a free, gamified CompTIA certification prep platform.

Why it fits this list:

• 100% free — no account required, works offline
• 1,500+ practice questions across Security+ (SY0-701), A+ (220-1101/1102), and Network+ (N10-009)
• AI-powered explanations for every question
• Gamified RPG learning — XP, levels, achievements, battle system
• PWA — works offline after first load

Added to the Training section.