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/src/lib/theme.ts

@@ -132,6 +132,18 @@ function applyToDocument(themeId: ThemeId, customCss: string | null): void {
   root.setAttribute('data-theme', attr);
   const light = customCss !== null ? isLightColor(readCssVar(customCss, 'bg-app')) : themeId === 'light';
   root.classList.toggle('dark', !light);
+  syncThemeColorMeta();
+}
+
+// The status bar (theme-color) starts blue for the splash screen; once the app runs it follows the
+// active theme's background so it blends with the header.
+function syncThemeColorMeta(): void {
+  const meta = document.querySelector('meta[name="theme-color"]');
+  if (!meta || typeof requestAnimationFrame === 'undefined') return;
+  requestAnimationFrame(() => {
+    const bg = getComputedStyle(document.documentElement).getPropertyValue('--bg-app').trim();
+    if (bg) meta.setAttribute('content', bg);
+  });
 }
 
 function store(key: string, value: string | null): void {