refactor: consolidate and debloat calendar sync implementation

Merge duplicated Google/Outlook engines, background handlers, and API
layers into shared calendarSync modules to cut complexity without changing
user-facing sync behavior.

jest.config.js

@@ -11,7 +11,6 @@ export default {
   },
   moduleNameMapper: {
     '^@/(.*)$': '<rootDir>/src/$1',
-    '^color$': '<rootDir>/src/test/mocks/color.ts',
     '^webextension-polyfill$': '<rootDir>/src/test/mocks/webextension-polyfill.ts',
   },
   moduleFileExtensions: ['ts', 'js', 'json'],