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/messages/UserMessage.tsx

@@ -6,6 +6,7 @@
 
 import React from 'react';
 import { Text, Box } from 'ink';
+import { Colors } from '../../colors.js';
 
 interface UserMessageProps {
   text: string;
@@ -18,7 +19,7 @@ export const UserMessage: React.FC<UserMessageProps> = ({ text }) => {
   return (
     <Box flexDirection="row">
       <Box width={prefixWidth}>
-        <Text color="gray">{prefix}</Text>
+        <Text color={Colors.SubtleComment}>{prefix}</Text>
       </Box>
       <Box flexGrow={1}>
         <Text wrap="wrap">{text}</Text>