style: overhaul design to authentic Google Gemini Material 3 Dark theme

frontend/src/index.css

@@ -3,23 +3,27 @@
 @tailwind utilities;
 
 :root {
-  /* BRAIN.md Strict Gemini Dark Theme Tokens */
-  --bg-app: #0b0f19;
-  --bg-surface: #131314;
-  --bg-card: #1e1f20;
-  --bg-card-hover: #282a2c;
+  /* Google Gemini / Material 3 Dark Palette */
+  --bg-canvas: #131314;
+  --bg-surface: #1e1f20;
+  --bg-surface-elevated: #282a2c;
+  --bg-surface-hover: #333537;
 
   --border-subtle: rgba(255, 255, 255, 0.08);
-  --border-focus: rgba(138, 180, 248, 0.40);
+  --border-medium: #3c4043;
+  --border-focus: #a8c7fa;
 
-  --accent-blue: #8ab4f8;
-  --accent-purple: #c58af9;
-  --swipe-keep: #81c995;
-  --swipe-discard: #f28b82;
+  --color-primary: #a8c7fa;
+  --color-primary-container: #004a77;
+  --color-on-primary-container: #d3e3fd;
+
+  --color-success: #a8dab5;
+  --color-danger: #f28b82;
+  --color-warning: #fdd663;
 
   --text-primary: #e3e3e3;
-  --text-muted: #9aa0a6;
-  --text-subtle: #5f6368;
+  --text-secondary: #c4c7c5;
+  --text-tertiary: #8e918f;
 }
 
 * {
@@ -28,13 +32,13 @@
 }
 
 body {
-  background-color: var(--bg-app);
+  background-color: var(--bg-canvas);
   color: var(--text-primary);
-  font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, sans-serif;
+  font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
   overflow-x: hidden;
 }
 
-/* Subtle custom dark scrollbar */
+/* Subtle Material scrollbar */
 ::-webkit-scrollbar {
   width: 6px;
   height: 6px;
@@ -50,20 +54,13 @@ body {
 }
 
 ::-webkit-scrollbar-thumb:hover {
-  background: rgba(138, 180, 248, 0.35);
+  background: rgba(255, 255, 255, 0.22);
 }
 
 .gemini-border {
   border: 1px solid var(--border-subtle);
 }
 
-.gemini-input-glow:focus-within {
-  border-color: rgba(138, 180, 248, 0.55);
-  box-shadow: 0 0 32px -4px rgba(138, 180, 248, 0.25), 0 0 12px -2px rgba(197, 138, 249, 0.2);
-}
-
-.gemini-gradient-text {
-  background: linear-gradient(135deg, #8ab4f8 0%, #c58af9 55%, #81c995 100%);
-  -webkit-background-clip: text;
-  -webkit-text-fill-color: transparent;
+.gemini-border-medium {
+  border: 1px solid var(--border-medium);
 }