Minimal flat redesign: white accent, remove card backgrounds

Replace yellow accent (#f5a623) with white (#ffffff) across the entire
app. Flatten VideoCard grid variant by removing zinc-900 card background
so content sits directly on the page. Simplify active states, badges,
progress bars, and hover effects throughout.

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

frontend/tailwind.config.js

@@ -10,9 +10,9 @@ export default {
       },
       colors: {
         accent: {
-          DEFAULT: "#f5a623",
-          light: "#fbbf45",
-          dark: "#d4891a",
+          DEFAULT: "#ffffff",
+          light: "#f4f4f5",
+          dark: "#d4d4d8",
         },
       },
       aspectRatio: {