feat(pwa): loading screen in the colours of the last used theme

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

frontend/src/lib/theme.ts

@@ -236,6 +236,7 @@ export function getAccentColor(): string | null {
 export function setAccentColor(hex: string | null): void {
   store(ACCENT_KEY, hex);
   applyAccent(hex);
+  syncThemeColorMeta(); // keeps the loading screen's colours current
 }
 
 function hexToRgb(hex: string): [number, number, number] {
@@ -352,15 +353,22 @@ function applyToDocument(themeId: ThemeId, customCss: string | null): void {
 
 // The status bar (theme-color) starts black (like the Android navigation bar of the installed app,
 // which only takes the manifest colour); once the app runs it follows the theme's background.
+// The colours are also kept for the next start: public/boot-theme.js paints the loading screen in them.
 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();
+    const css = getComputedStyle(document.documentElement);
+    const bg = css.getPropertyValue('--bg-app').trim();
     if (bg) meta.setAttribute('content', bg);
+    const accent = css.getPropertyValue('--accent-primary').trim();
+    const onAccent = css.getPropertyValue('--text-on-accent').trim();
+    if (bg && accent) store(BOOT_COLORS_KEY, JSON.stringify({ bg, accent, onAccent }));
   });
 }
 
+const BOOT_COLORS_KEY = 'mindsift_boot_colors';
+
 function store(key: string, value: string | null): void {
   try {
     if (value === null) localStorage.removeItem(key);