fix: splash screen hangs after login, never dismisses

Root cause: showSplash() sets display:flex + opacity:1 synchronously,
then dismissSplash() immediately adds the fade-out class (opacity:0).
The browser batches these in the same paint frame so the CSS transition
from opacity:1 -> 0 never starts, and transitionend never fires,
leaving the Promise unresolved and the splash stuck.

Two-part fix:
1. handleLogin: await two requestAnimationFrames between showSplash()
   and dismissSplash() so the browser paints opacity:1 first, ensuring
   the CSS opacity transition actually runs.
2. dismissSplash: add a 500ms fallback setTimeout that hides the splash
   and resolves the Promise even if transitionend is never fired (acts
   as a safety net for any future edge cases).

public/app.js

@@ -99,7 +99,15 @@ function dismissSplash(startTime) {
     setTimeout(() => {
       const splash = document.getElementById('splash-screen');
       splash.classList.add('fade-out');
+      // Fallback: resolve after transition duration + buffer in case
+      // transitionend never fires (e.g. display was toggled in same frame)
+      const TRANSITION_MS = 400;
+      const fallback = setTimeout(() => {
+        splash.style.display = 'none';
+        resolve();
+      }, TRANSITION_MS + 100);
       splash.addEventListener('transitionend', () => {
+        clearTimeout(fallback);
         splash.style.display = 'none';
         resolve();
       }, { once: true });
@@ -152,9 +160,13 @@ async function handleLogin(e) {
     if (data.success) {
       currentUser = data.user;
       isAdmin = !!data.user.isAdmin;
-      // Fade out login, then show splash while loading data
+      // Fade out login, then show splash while loading data.
+      // requestAnimationFrame ensures the browser paints the splash at
+      // opacity:1 before dismissSplash adds fade-out, so the CSS
+      // transition fires and transitionend is guaranteed.
       await fadeOutLogin();
       showSplash();
+      await new Promise(r => requestAnimationFrame(() => requestAnimationFrame(r)));
       showDashboard();
       const splashStart = Date.now();
       await fetchUserDownloads(true);