feat(ui): add vertical logo lockup splash screen on app load

Introduce a full-screen brand splash shown while the project store is
loading (status !== ready). Uses a new vertical logo lockup (P tile /
PubScript wordmark / S tile) matching the existing typewriter-tile
design, centered on the active theme background with a subtle
fade/appear animation and a graceful fade-out once the app is ready.

- apps/web/public/logo-lockup.svg: vertical lockup variant of the mark
- SplashScreen.tsx: overlay driven by the store's load status
- splash.css: full-screen overlay + keyframe animations
- wire into App.tsx and import splash.css in main.tsx

apps/web/src/main.tsx

@@ -9,6 +9,7 @@ import "./styles/plot.css";
 import "./styles/timeline.css";
 import "./styles/whiteboard.css";
 import "./styles/export.css";
+import "./styles/splash.css";
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>