feat: Start refactoring code base to be testable

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

src/sheetUtils.test.ts

@@ -0,0 +1,5 @@
+describe('sheetUtils', () => {
+    it('should be able to run tests', () => {
+        expect(true).toBe(true);
+    });
+});