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/LoadingIndicator.tsx

@@ -7,6 +7,7 @@
 import React from 'react';
 import { Box, Text } from 'ink';
 import Spinner from 'ink-spinner';
+import { Colors } from '../colors.js';
 
 interface LoadingIndicatorProps {
   isLoading: boolean;
@@ -28,11 +29,11 @@ export const LoadingIndicator: React.FC<LoadingIndicatorProps> = ({
       <Box marginRight={1}>
         <Spinner type="dots" />
       </Box>
-      <Text color="cyan">
+      <Text color={Colors.AccentPurple}>
         {currentLoadingPhrase} ({elapsedTime}s)
       </Text>
       <Box flexGrow={1}>{/* Spacer */}</Box>
-      <Text color="gray">(ESC to cancel)</Text>
+      <Text color={Colors.SubtleComment}>(ESC to cancel)</Text>
     </Box>
   );
 };