Add speed and fire monster variantsfeat: add new monster variants

[whyer123]

Summary

• Add SpeedMonster with charge warning and fixed-direction dash attack
• Add FireMonster with burn damage on hit
• Increase base monster HP from 5 to 50
• Add SpeedMonster and FireMonster spawn scenes and assets
• Update monster spawning probabilities:
  • SpeedMonster: 1/10
  • FireMonster: 1/5
  • Normal Monster: 7/10

Testing

• Ran dotnet format --verbosity diagnostic QuestFantasy.sln
• Ran dotnet build QuestFantasy.sln
• Confirmed build passed with 0 warnings and 0 errors
• Ran git diff --check