Fix accent color flash on refresh by eliminating CSS/JS race

Guard useTheme effect to skip when settings are undefined, preventing
it from overwriting the inline script's cached color with cyan defaults.
Move CSS accent var defaults from index.css :root into the index.html
inline script so they are always set synchronously before paint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/index.html

@@ -9,18 +9,24 @@
     <meta name="mobile-web-app-capable" content="yes" />
     <title>UMBRA</title>
     <script>
-      // Apply cached accent color before React hydrates to prevent cyan flash
+      // Apply accent color before React hydrates to prevent FOUC.
+      // Reads from localStorage cache; falls back to cyan on first visit.
       (function() {
+        var s = document.documentElement.style;
         try {
           var c = localStorage.getItem('umbra-accent-color');
           if (c) {
             var p = JSON.parse(c);
-            var s = document.documentElement.style;
             s.setProperty('--accent-h', p.h);
             s.setProperty('--accent-s', p.s);
             s.setProperty('--accent-l', p.l);
+            return;
           }
         } catch(e) {}
+        // First visit or corrupt cache — apply cyan defaults
+        s.setProperty('--accent-h', '187');
+        s.setProperty('--accent-s', '85.7%');
+        s.setProperty('--accent-l', '53.3%');
       })();
     </script>
     <link rel="preconnect" href="https://fonts.googleapis.com" />

frontend/src/hooks/useTheme.ts

@@ -15,10 +15,13 @@ const ACCENT_PRESETS: Record<string, { h: number; s: number; l: number }> = {
 export function useTheme() {
   const { settings } = useSettings();
 
-  // Ensure localStorage always has an accent color (even default cyan)
-  // so the inline script in index.html can prevent flashes on every load
+  // Only apply accent color once settings have loaded from the API.
+  // The inline script in index.html handles the initial paint from localStorage cache.
+  // Firing this effect with settings=undefined would overwrite the cache with cyan.
   useEffect(() => {
-    const colorName = settings?.accent_color || 'cyan';
+    if (!settings) return;
+
+    const colorName = settings.accent_color || 'cyan';
     const preset = ACCENT_PRESETS[colorName];
     if (!preset) return;
 
@@ -31,7 +34,7 @@ export function useTheme() {
     try {
       localStorage.setItem('umbra-accent-color', JSON.stringify({ h, s, l }));
     } catch {}
-  }, [settings?.accent_color]);
+  }, [settings]);
 
   return {
     accentColor: settings?.accent_color || 'cyan',

frontend/src/index.css

@@ -26,10 +26,9 @@
     --ring: var(--accent-h) var(--accent-s) var(--accent-l);
     --radius: 0.5rem;
 
-    /* Default accent: cyan */
-    --accent-h: 187;
-    --accent-s: 85.7%;
-    --accent-l: 53.3%;
+    /* Accent vars are set by the inline script in index.html (from localStorage
+       cache or cyan fallback). No CSS defaults here — they would race with the
+       inline script and cause a flash of wrong color on refresh. */
 
     /* Transitions */
     --transition-fast: 150ms;