feat: Start refactoring code base to be testable

Implement a spreadsheet service abstraction, GAS integration, and Jest testing setup.

jest.config.js

@@ -0,0 +1,6 @@
+module.exports = {
+  preset: 'ts-jest',
+  testEnvironment: 'node',
+  testMatch: ['**/*.test.ts'],
+  setupFiles: ['<rootDir>/src/test/setup.ts'],
+};