Give Gemini Code a face lift.

- This utilizes `ink-gradient` to render GEMINI CODE in amazing colors.
- Added a shared color configuration for UX (should this be in config?). It's very possible that we shouldn't be talking about the specific colors and instead be mentioning "foreground"/"background"/inlineCode etc. type colors.
- Updated existing color usages to utilize `Colors.*`

Fixes https://b.corp.google.com/issues/411385593

packages/cli/src/ui/components/InputPrompt.tsx

@@ -5,8 +5,9 @@
  */
 
 import React from 'react';
-import { Box, useInput, useFocus } from 'ink';
+import { Text, Box, useInput, useFocus } from 'ink';
 import TextInput from 'ink-text-input';
+import { Colors } from '../colors.js';
 
 interface InputPromptProps {
   onSubmit: (value: string) => void;
@@ -29,19 +30,18 @@ export const InputPrompt: React.FC<InputPromptProps> = ({ onSubmit }) => {
   );
 
   return (
-    <Box
-      borderStyle="round"
-      borderColor={isFocused ? 'blue' : 'gray'}
-      paddingX={1}
-    >
-      <TextInput
-        value={value}
-        onChange={setValue}
-        placeholder="Enter your message or use tools..."
-        onSubmit={() => {
-          /* Empty to prevent double submission */
-        }}
-      />
+    <Box borderStyle="round" borderColor={Colors.AccentBlue} paddingX={1}>
+      <Text color={Colors.AccentPurple}>&gt; </Text>
+      <Box flexGrow={1}>
+        <TextInput
+          value={value}
+          onChange={setValue}
+          placeholder="Enter your message or use tools..."
+          onSubmit={() => {
+            /* Empty to prevent double submission */
+          }}
+        />
+      </Box>
     </Box>
   );
 };