feat(pwa): blue splash screen with the logo, status bar follows the theme

Manifest: short name, blue background/theme colour for the Android splash.
index.html shows a plain blue screen with the logo until React mounts.
theme-color starts blue and then follows the active theme background.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/index.html

@@ -3,7 +3,8 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no" />
-    <meta name="theme-color" content="#0b0f19" />
+    <!-- Blue while loading (matches the splash screen); the app switches it to the theme background -->
+    <meta name="theme-color" content="#2563eb" />
     <meta name="mobile-web-app-capable" content="yes" />
     <meta name="apple-mobile-web-app-capable" content="yes" />
     <meta name="apple-mobile-web-app-title" content="MindSift" />
@@ -49,7 +50,13 @@
     </script>
   </head>
   <body class="bg-[#0b0f19] text-[#e3e3e3] antialiased selection:bg-[#8ab4f8]/30 selection:text-[#8ab4f8]">
-    <div id="root"></div>
+    <!-- Shown until React mounts and replaces it: full blue with the logo, no text, no white flash -->
+    <div id="root">
+      <div style="position:fixed;inset:0;background:#2563eb;display:flex;align-items:center;justify-content:center">
+        <!-- the icon's own blue square disappears on blue, so the visible brain is ~55% of this size -->
+        <img src="/icon.svg" width="128" height="128" alt="" style="display:block" />
+      </div>
+    </div>
     <script type="module" src="/src/main.tsx"></script>
     <script>
       if ('caches' in window) {