Fix sprite transparency: remove purple background from Mario sprite sheet

[qa-philipp-bauhofer]

Summary

• The assets/sprite.png sprite sheet used opaque colored backgrounds (three color variants: #002980, #0000A8, #9290FF) as a legacy NES-era color-key convention instead of proper alpha-channel transparency
• This caused a visible purple rectangle to render around Mario whenever he walked in front of trees or other background elements
• Fixed by applying a flood-fill algorithm starting from the image boundary to remove all connected background regions while preserving internal sprite pixels

What changed

assets/sprite.png — background pixels replaced with alpha=0 (transparent):

• First pass: exact color-match on corner color (0, 41, 140) → 113,072 px transparent
• Second pass: flood-fill from image boundary covering all three background color variants → 68,118 additional px transparent
• Total: ~181,000 px made transparent out of ~274,000 total

What a reviewer should know

• The flood-fill approach was chosen over global color replacement to avoid accidentally removing valid sprite pixels that happen to share a background color (a risk with NES palette sprites that reuse colors)
• The palette reference panels in the lower-right section of the sprite sheet retain their colored backgrounds — these are informational annotations by the sprite sheet author and are not used by the game
• The sprite regions used by Player.ts (idle + 3 walk frames at y=8) were verified post-fix: ~40–52% of each cell is now transparent, consistent with character sprite shapes

Test plan

• Run npm run dev and open http://localhost:8000
• Walk Mario left/right past background trees — no purple rectangle should appear around the character
• Verify idle and all 3 walk animation frames render without background artifacts

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