feat: mobile Julia set random init + scroll morphing

[debuggingfuture]

Summary

• On mobile viewports (<768px), the Julia set background now initializes with a random coordinate from 8 curated visually interesting constants instead of the fixed default
• As the user scrolls, the fractal smoothly interpolates between preset coordinates, creating an evolving background effect
• Desktop behavior is unchanged — mouse movement still controls the fractal

Test plan

• Open on mobile (or devtools responsive mode < 768px) — verify the fractal starts with a random interesting shape (refresh a few times to see variation)
• Scroll the page on mobile — verify the fractal morphs smoothly between different shapes
• Open on desktop (>= 768px) — verify mouse-driven control still works as before
• Check that scroll listener uses passive: true and doesn't degrade scroll performance