feat: add Three.js splash screen on app startup

[j4rviscmd]

Summary

Add a full-screen animated splash screen that displays during app initialization. The splash shows a constellation particle field with a slowly rotating wireframe icosahedron behind the "Scripta" wordmark.

Changes

New files (6)

• src/features/splash/lib/constants.ts — Configuration constants (particle count, colors, timing)
• src/features/splash/lib/createScene.ts — Pure Three.js scene factory (particles + connection lines + wireframe icosahedron)
• src/features/splash/hooks/useSplashLifecycle.ts — Lifecycle state machine (active → fading → done)
• src/features/splash/hooks/useThreeScene.ts — React ↔ Three.js imperative bridge with dynamic import
• src/features/splash/ui/SplashScreen.tsx — Full-screen overlay component with fade-out transition
• src/features/splash/index.ts — Public API barrel export

Modified files (2)

• src/app/providers/store-provider.tsx — Export storeInitPromise for splash timing coordination
• src/main.tsx — Add <SplashScreen /> as overlay sibling

Dependencies

• Added three and @types/three

Architecture

Follows the Clean architecture pattern with clear separation of concerns:

features/splash/
├── index.ts              # Public API
├── ui/SplashScreen.tsx   # React overlay + text
├── hooks/
│   ├── useSplashLifecycle.ts  # Timing state machine
│   └── useThreeScene.ts      # Three.js bridge
└── lib/
    ├── constants.ts      # Configuration
    └── createScene.ts    # Pure Three.js scene

Behavior

1. Splash appears immediately on app launch (dark background + "Scripta" text)
2. Three.js constellation loads via dynamic import (~50ms)
3. Waits for both: store initialization AND minimum 2-second display
4. Smooth 700ms opacity fade-out
5. Component unmounts, Three.js resources fully disposed

Technical Notes

• Three.js is code-split via dynamic import() — separate Vite chunk, not in main bundle
• StrictMode-safe: cleanup guards prevent double-creation
• Full GPU resource cleanup on dispose (geometries, materials, renderer)
• Zero changes to Rust backend