feat: add animated forge/factory hero background with particle effects, molten glow, and smoke wisps (Closes #850) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Summary

Upgrades the hero section background from a simple grid + 5 ember particles to a rich forge/factory-themed animated background.

What's Added

• SparkShower — 40 animated spark particles that fly upward with random drift, opacity, and timing
• EmberDrift — 15 slow-drift ember particles with scaling and fading
• MoltenGlow — Animated molten core glow at the bottom with ripple effect
• SmokeWisps — 6 slow-rising smoke wisps for atmospheric depth
• ForgeSilhouette — Decorative SVG anvil/gear shapes at the bottom with rotating gear animations

Performance

• Pure CSS animations — no canvas, no requestAnimationFrame
• All animations use GPU-composited properties (transform, opacity)
• Minimal bundle size impact (< 1KB for the component, negligible CSS additions)
• Smooth 60fps on modern browsers

Acceptance Criteria

• Smooth animations at 60fps on modern browsers
• Responsive to different screen sizes
• Minimal performance impact (< 30MB bundle size)

Technical Details

• New component: ForgeBackground.tsx (React + CSS animations)
• 4 new CSS keyframe animations: spark, ember-drift, molten-ripple, smoke
• Uses framer-motion compatibility — no breaking changes
• Responsive: all positions are percentage-based, SVG scales to container width