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/App.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect } from "react";
 import { Routes, Route } from "react-router-dom";
-import { Sidebar, Topbar, CommandPalette } from "./components/index.js";
+import { Sidebar, Topbar, CommandPalette, SplashScreen } from "./components/index.js";
 import { ExportDialog } from "./features/export/ExportDialog.js";
 import { ToastViewport } from "@pubscript/ui";
 import Write from "./routes/Write.js";
@@ -42,6 +42,7 @@ export default function App() {
 
   return (
     <div className="ps-app">
+      <SplashScreen />
       <Sidebar nav={NAV} />
       <Topbar onOpenPalette={() => setPaletteOpen(true)} onOpenExport={() => setExportOpen(true)} />
       <main className="ps-main">