Refactoring to enable testability of popup.js

[evanwon]

Root Causes Requiring Refactoring:

1. popup.js initialization issue: The file runs initialization code at load time (lines 100-108 in
   popup.js). This should be refactored to export functions that can be tested independently.
2. Test isolation issue: The jest/jsdom environment isn't properly cleaning up between tests, causing
   document to become undefined. This needs better test setup/teardown.
3. Module caching issue: The delete require.cache approach (line 71) isn't working correctly, causing
   state to leak between tests.

● Summary

Passing Tests Analysis:

• 26 tests are currently passing
• Most passing tests are valid and should be kept
• They test core functionality: UI generation, tab management, settings, copy operations

Failing Tests Analysis:

• 15 tests are failing due to two main issues:
  a. popup.js runs initialization code when loaded, causing side effects
  b. jsdom environment corruption between test runs

Recommendations:

1. Keep all 26 passing tests - they're testing valid functionality
2. Remove 15 failing tests temporarily until refactoring addresses:
   - Separating popup.js initialization from function definitions
   - Fixing test environment isolation issues
   - Properly handling module caching in tests

The failing tests themselves are conceptually valid but need the underlying code structure to be
refactored to be testable. The main issue is that popup.js wasn't designed with testing in mind - it
immediately executes code on load rather than exporting testable functions.