Fix Unicode character handling in API responses

[solavrc]

Problem

When the backend returns responses containing Unicode characters (like Japanese), the Looker proxy encounters character encoding issues and returns 500 errors to the frontend.

Solution

This PR implements a base64 encoding/decoding approach to safely transmit Unicode characters:

• Backend: Encode JSON responses as base64 strings before sending them through the Looker proxy
• Frontend: Add a utility function that decodes base64 responses with proper UTF-8 handling
• Ensure multi-byte characters are correctly processed using TextDecoder

Implementation Details

1. Modified backend endpoint handlers to encode responses using Buffer.from(JSON.stringify()).toString('base64')
2. Created base64Helper.ts utility for frontend to decode responses with Unicode support
3. Updated API utility functions to use the base64 decoder for all responses

This approach ensures consistent handling of all character sets, including Japanese, without relying on Looker proxy's character encoding capabilities.

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[solavrc]

The webpack configuration explicitly disables Buffer polyfill with:

dashboard-summarization/webpack.config.js

Lines 76 to 78 in af034be

	resolve: {
	extensions: [".tsx", ".ts", ".js"],
	fallback: { buffer: false },